Bob,
Another possibility is to put an ad in Aquaculture magazine (828) 254-7334 or Fish Farming News (207) 367-2490. I don't know what it would cost, but it may not be too much. Even if it is getting late in the season you might be able to make a contact for next year.
I once caught a 3 lb yellow bullhead (I know what they are as I have a fisheries degree) in a local lake, but it was the only one I ever caught. Going out and trying to ger more more would be hard to do.
Would you still be interested next year? I like challenges, and if I found a local lake that has plenty I could put them in a floating cage. The fish would be free, but shipping them alive would be the killer. If I had an extra pond the easiest thing would be to allow the females to spawn and net up the fry and ship them air.
I hope you have someone on this site that has a pond with yellows in it. Then they could scoop net the fry up and ship.
Oh John, I think most, if not all, of the pay lakes use channel catfish anyway as they are easier to find for stocking.
Good Luck Bob