You might be best off if you select the casinos you want chips from first. Harrah's is a good bet because there are Multiple Harrah's Casinos.

Most of the time people will sell you a current rack chip for a small premium over face value.

Problem is that if you pick a place with only one or two locations you will have a hard time getting 20-50 current chips. Once you start getting into the obsolete chips it becomes more difficult to gauge the cost because values vary widely.

Once you select your casino, post your request for chips (specify denominations) and I'll bet that you get a bunch of responses.