They don't need to be Fidelipac/Dynamax. Look out also for Capitol Audiopak, 3M Scotchcart, Sonifex Sapphire, Cartridge Technology etc. Given that the US is the home of the NAB cartridge, I am surprised that you are having trouble finding them. (Mind you, I've never seen an actual Muntz 4-track cartridge per se.)
A few things to beware of:
1. Audiopaks came in several versions, the most recent being the AA-4 (I think) which had a transparent blue top.
2. The foam pads go bad, resulting in poor tape-to-head contact. But there was an outfit called Cart Guys (no idea if they still exist) who could supply pads.
3. Changing the pads is not easy.
4. Scotchcarts don't have pads!
5. AA-size carts run to about 10 minutes max at 7.5 i.p.s., but your machine can possibly take the next size up (somebody please remind me of the nomenclature!) although expect those to be unobtainium.
6. There are better machines, to be got for nowt. A friend recently junked some PR&E Tomcats, which in their day were seriously good.
7. Most machines don't have an erase head - and in fact replay-only machines have a dummy head (where the record head might be) to act as a tape guide. One notable exception was the ITC Series99 which not just erased, but located the splice also.
8. Make friends with retired radio station engineers. Chances are they'll have some carts that they'll be glad to give you, and possibly even service your machine for free, just for fun.
R