I am looking for a bus bar for a Challenger SL12 sub panel. Any ideas?

Yeah, replace the whole panel.

I think you chances of finding a buss only for an old Challenger panel are near zero.

What I expected to hear but was hoping otherwise. Thanks.

Seriously, have you searched eBay?

In order to replace the buss, you have to remove everything anyway... changing the panel at that time won't be a big deal. Do it safely.