I have found that casinos may not nessessarily let you paw through their inventory, but most are helpful enough to go through themselves and pull out as many different chips as you are willing to buy, then let you choose which ones you want from there. When I was at the hard rock a few months ago, I had a list of the ones I was missing and the cashier was more than willing to search through her drawer to see if she had any from that list, as well as ask the cashier next to her.

Try to go during a slow time. You'll get a lot more friendly cashiers willing to help you out mid week at six or seven in the morning than Saturday at 6 or 7 in the evening. Good luck on your hunt.