Ammunition? For the 90mm in the M26/46/47/48 & M56 Scorpion (threw that one in for laughs), you might get some practice and a small quanitity of semi-moldy live rounds from some storage site..somewhere. Those familiar with them could comment bettor on that.
For most anything alse, the ammunition would be largely home brewed, probably using the services of local chemists and machinists.
That includes the 75mm and 76mm from the M4/M18 and 3 inch from the M10, 75mm from the M3 GMC, and 75mm from a variety of WW2 German pieces (except for the L70 on the Mark 5 and JgPz IV/L70, all ballistically similar).
Think it can't be done? Shaped charge munitions are nothing new and industrial/military explosives can and will be located.
In many cases, that will involve disassembling military rounds for other ordnance, most likely. Not a job I'd want, but desperate times...
Check this out:
http://www.youtube.com/watch?v=CvncpT4EVzQ
That's a 90mm T8 AT Gun..most of the footage I've seen involves solid shot the owner made himself using old brass casings.
There are 57mm M1/6pdr AT guns out there firing solid shot too. They had one on "Lock n Load with R.Lee Ermey"
For what you need to do...knock out some bank armored car or M-113, you don't need tungsten core anything at these calibers....4140 pre-hardened or even soft will do the job just fine.
An example and/or cutaway training round would work well as an example for sizes of the projectile.
Before some of you have a fit over some outfit/municipality having a pile of ammo for their 57mm/75mm/90mm because they have a source for explosives and a talented machine shop, owners of such guns (self propelled or towed) would be certainly limited in how many casings and therefore rounds they could make.
Unless you have some collector with a full load of deac 37mm rounds for their M5 Staurt / M8 Greyhound for show putposes....seen that at a show at least once.
I see no situation where a town could just shell a biker gang willy-nilly. It would be fun to watch, though.
Not was much fun as a quad 50 chopping them to bits though, but that's just me!
However, I really wouldn't want to be some NA kook ordering his armored car(s) up only to see it(them) take 90mm steel solid shot right through the grill. Ouch.
Of course, sourcing ammunition (90mm cannon or 105mm howitzer most likely) or parts for a cannon or tank could be a nice mini adventure in itself.
Are live cannons from private/museum sources possible in the US? Umm, yes.
Are they more of a threat because all you've got are a dozen or so rounds for it? Yes to that as well.
My $.05
Dave