When you arrive, the first thing you should ask is if they have any old casino gaming chips. Even if there are none out for sale, sometimes people will remember a couple they brought back home and just tossed in a drawer!
If they don't have any chips, ask about other gaming related items. Even if you don't collect anything other than chips. If you get something else cheap enough, you might be able to trade it for chips in the future. Or sell it, if the item is something special.
Also, make sure to leave your contact information with the person putting on the garage sale. You never know when they might meet someone who has some chips.