Actually the best fit for "Heroic 28mm scale" outside of models made with heroic scale in mind are simple non-scale toyatic style stuff like MENG's Toon Tank line or the Buzz Lightyear toy vehicles since heroic isn't really a scale so much as a style of mini-sculpting (a rather cartoony style I might add that was designed to sell to kids originally 😆). Basically if the hatches/doors it comes with or you have added look like they could fit the "heroic scale" bodies your working with believably to you, then it works. Vehicles can be anything from a sold as 1/24 car kits to a 1/144 mecha as long as it looks like it could fit in with your heroic scale figures, (and yes people have moded 1/72 vehicles into heroic scale ones😉).
You can even use army men tanks and trucks, I will list these three that seem to be among the cheapest options outside of dumpster diving that I have heard “good things about” in my Facebook groups and elsewhere occasionally (I wanted to list them earlier but I could not find them for sale anywhere till after a long hunt).
You will likely want was wash and do a ton of conversion work to these before painting them, and I will warn I am using Aliexpress (which is not noted for safe shipping) for this because USA Amazon and Ebay don’t seem to carry all of these by themselves anymore (UK/EU/AU/Canada/elsewhere might be a different story), anyway now for some blue links:
First we have the highly recommended cheap Chinese plastic double barreled light tanks/tankette (nicknamed the Shoddy by some) they also seem to work for more historical scale VBCW gaming as well. This is a 10 to a pack link here
Then there is the armored APC based loosely on a real Chinese APC. It is rather large by 28mm standards (I have seen it billed as being 1/24), but I have heard of it being used for 28mm war gaming (my guess is as some larger more fantastical vehicles in 40k). Link here
And finally there are these “not rhinos” that supposedly work as you guessed rhino tank proxies, they do seem to be big enough for that and at least don't look like WW1 tanks. Here is a link to those.
There is also this truck which you can get on ebay still in 10 packs , but they seem to be better for more true to life historical scale minis.
Now, those are not the best options clearly, but they are among (if not) the cheapest, again meng toon tanks and Technolog are probably the best inexpensive with decent quality options for most Death fields force needs outside of STL land though both have issues (one is chibi WW2 tanks and just the chibi WW2 tanks except for a rare Chibi APC kit based on a Chinese SciFi film, while the other is Russian based meaning you can’t get them except by lucky ebay finds).
For slightly better quality that is still cheap I have seen people convert paw patrol, Buzz Lightyear and other action figure vehicles to satisfy their heroic scale motor pool needs. The Buzz Lightyear truck and drop ship in particular have been making the rounds in conversion circles, the truck even has tutorial on the Malstrom Edge website blog.(by the way you should really check that blog out).
There is also a sea of 1/50 to 1/24 diecast vehicles ranging from cars, construct and normal army vehicles to things such as the tank like Batmobiles. Some work, some don't, some work better for historical scale 28mm stuff like the Ironcore line.
Now there are a couple of "made for heroic scale model" sites that sell "cheap" parts in addtion to full models that would be useful if one is going this route. I am going to mention Ramshackle Games and Zing Industeries, neither are the world's cheapest presay, but they are still good deals over all and Ramshackle has a wide selection of parts and vehicles that would work great with the upcoming Damned theme wise.
I am going to list/mention a couple of other useful links that lead to other forums and blogs, since we are far from the only ones that have talked about this before.
Dakkadakka has at least two older threads related to the topic of heroic scale vehicles. One is focused on mecha kits/toys useful for proxying imperial knights, etc.. The other thread is more about detailing and general conversions (so not just vehicles), this one has a lot of dead threads to companies that I know are still out there, so it might be wiser to cut and paste the names of the ones that interest you into a search engine. You can also check out the gallery on that forum, massive collection converted vehicles a good number of which have referances to where they got what.
Chicago Skirmish is a long running blog that appears to be tied to a club, they have lots of helpful hints and suggests for doing your wargame hobby on the cheap.
Oh, and somewhat unrelated to my comment but related to this thread, found this cool MDF kit maker called Minature Scenery that has a ton of SciFi vehicles.
