Unfortunately, SS tank parts are weirdly unique to their original brethren. If you have a ss tank, tank lid, and tank lid cap, they are only ever going to fit properly with each other. You start mixing them up and things won't go on, or won't stay on, or will leak, etc. So even if you find someone with 5 loose caps to sell you, it's not going to be easy to make any of them actually fit. And this is why you're not going to find a source of new replacement caps: there's no point.
So your best bet at this point is going to be finding a source for those black plastic caps you see on some tanks. Those can stretch to fit any tank lid more or less.
Duncan