when I'm making my rounds I've found that you have to be persistent.
Ask "do you have any casino CHIPS."

LET THEM MAKE THEIR JOKES.
Ask again.

after they say no the second time. I say.
"If its one that I need I'll pay more than face value for it." if they have any that usually works.
Also if you ask for other memorabilia in addition to chips you may start up a conversation that will lead you to someone else who has chips.
By the way I have first dids on any photos you may find in your wanderings.