Tamiya's 1/48 modern armor kits are fine from what I've seen: scaled pretty much identically to WGA's 28mm guys. That Tamiya Japanese armored car and Hummer look great with WGA's Iron Core and Death Fields figures!
The Tamiya M8 mortar might be just a dash too big, if the included soldier figure is anything to go by (he's nearly a head taller than many 28mm figures - not very far out of the range or normal human height differences), but if you're pitching the figure and just converting the vehicle to sci-fi you'll be fine. That turret looks a bit dated, but with a little creative modeling you could probably scratch-build something a little more futuristic onto it, and replace the main gun and .50 calibre machine-gun with a Gatling-style mini-gun or something (there are 1/48 scale add-on mini-gun barrels made for C-47 gunship models available from Quickboost.)
Your mileage might vary with other model companies, but from what I've seen of Ukraine's ACE 1/48 stuff, it's also pretty much spot-on, too, though I'm not very impressed with the quality of the sample I'm working with - the parts do not fit together well at all. Still, ACE's FAI-M armored car is one of the few Russian non-WWII tank plastic model kits I've seen still available in this scale, and it wasn't very expensive, so if you don't mind a little patching-and-filling or scratch-building to get something from it, then you're in for a bit of an adventure, and the subject is visually pretty interesting:
I'll be souping my sample up into something loosely inspired by the Creeper's "Vehicle of Choice for A-holes and Effing Serial Killers" from the horror movie Jeepers Creepers - since I'm going to scratch-build new side panels and rooftop anyway, it shouldn't be a whole lot more work to add on an enclosed truck bed and raise the roof a bit to accomodate the driver (he'll be a plastic Undead Outlaw from Reaper)... the post-apocalyptic mutant cannibal Russian hillbilly thing kinda runs perfectly with my vision of the Damned:
(Heavily modified FAI-M armored car converted to a "Creeper", left, posed with a 1/72 Fennek converted with 28mm driver and gunner; painting is not finished yet on either, and I still need to add windows to the Fennek.)
This kit comes with a few optional parts that build a rounder hood and front that looks very similar to the Creeper truck, which is a nice touch, and I'll replace the front grill with a scratch-built thing made from styrene I-beams and grill, and I think I'll drop the turret and maybe try to find a way to bolt-on an old-fashioned supercharger/blower, and teach myself how to do the weathering and rust effects - it won't be a perfect match for the movie truck, of course, but it'll be fine for my vision of a broad-strokes, vaguely Russian alternate-universe version, as an command/ambulance/chuckwagon sorta thing.
This ACE Russian pickup might have been fun (and probably shares some of the sprues with the FAI-M), but it was out of stock:
This Tamiya Russian military truck is also out of stock, but looks like it could have been cool:
If you want to do anything with T-34s or other WWII Russian tanks, you're in a bit more luck.
The Tamiya Japanese Type 16 isn't a bad-looking modern vehicle in 1/48; with a little scratch-building creativity, some styrene sheets and other odds-and-ends, and a little work, I think the sides can be raised, the turret replaced with a remote machine-gun, and the back panel replaced with a door/ramp to convert to a Boxer or Vilkas-style AFV:
("Type 16" Model kit as designed, "Boxer" reference photo, and model converted to APC, staged with Ooh-Rah guys by Wargames Atlantic and power armor figure by Reaper Bones; painting is not complete yet but I'm most of the way finished.)
I picked up an Italeri 1/56 M8/M20 armored car, it's not a very common sight and looks futuristic enough to disguise as a weird future combat vehicle of some sort, but I find this 1/56 scale vehicle to look just a dash on the small side compared to 28mm - the included driver figure is just half a torso that sits on top of the seat, and still barely fits in the vehicle! Weird, and 28mm guys on plastic bases will dwarf this vehicle (the artist's painting of the vehicle doesn't really do justice to the kit contents), but if you can overlook that, the goofy half a driver figure can easily be head-swapped with a Death Fields head, and some creative kit-bashing can probably replace the old WWII style turrets and weaponry with something a little more space-age:
There aren't many model kit options at all out there in 1/48 scale, and even fewerin 1/56 scale, I'm afraid.
Beyond these sorts of 1/48 and 1/56 plastic model kits (most of them WWII era), I think you're stuck with four choices:
- 3D printing
- Cardstock projects (I've done this once before with a WWI tank that I printed on a bit of cereal box, it was a lot of fun, actually, and didn't look too bad)
- Raiding Games Workshop, Archon Studios, Manic, and other catalogues for 28mm sci-fi vehicles (including the new modular vehicle that is being developed by Wargames Atlantic for The Damned). This isn't a bad option, and I think WGA's entry looks promising, but honestly I just don't care for the cartoony 40K style stuff from GW and some of those other manufacturers, and it's kinda what EVERYONE seems to default to. (YMMV, of course! You take what you can get, either way.)
- Taking your chances trolling the toy aisles of your local dollar stores or S-Marts for toy vehicles, some of which are roughly 28mm scale, and can vary from fairly realistic stuff to hilariously cartoonish toys, probably in cheap plastic and chrome that would need to be given a makeover to look a little more serious. I can't make any recommendations here - good luck!
Anyway, don't let hte 1/48 thing fool you - it's a lot closer to 28mm heroic scale than you might think, and works just fine for being modified into sci-fi projects. (Wish I could say the same thing for 1/35 scale armor or 1/24 scale civilian car kits, both of which are available in a big variety of options, and both of which are way, way way too big to handwave for 28mm projects!)