Mike, yes there were a lot of caps available in St. Louis, and if my memory (of what I read) serves me right, there was quite a few that made their way up to Ft. Union.
Having them on hand and selling them however, I don't know if many would be that axnious to switch from what they were using? I think it was one of the Henry's that made an attempt at a percussion Northwest gun sometime in the early 1830's that did not do well at all. They couldn't sell/trade it to the Indians as the Indians preferred their flintlock Northwest guns.
Eventually the percussion system caught on and during its introduction time there were flintlocks converted to caplocks. In what numbers I think would be anyones guess? But it did happen.
My feelings are that yes, there was the potential of a percussion cap zenith trade to be had in St. Louis and up at Ft. Union in the 1830's, but my guess is that it was a slower conversion prossess that didn't take a real hold until the opening of the Oregon Trail.
That's not to say caplocks weren't at Rendezvous, as we know they were. I just don't think they were there in great numbers.