In OC there is a place called Orange County circuit breaker that has salvage parts. Maybe they could ship to you.
In the past I have had to retrofit an existing panel with a new buss with more breakers available. I first called on the inspector to make sure that he would not flag me for not having enough room in the panel. I then bought new panels and removed the buss and discarded the empty panel can.i thought i read here once that it was possible to buy just the bus bar etc., to retrofit an existing panel that would be a big pain to replace. I asked my supply house and they couldn't help me. Is there an online source?