Where can you buy a clothes rack for a thrift shop?

Since you're a retailer, you'll want to buy wholesale. Try http://www.robertham.com/c-952-retail-clothing-racks.aspx for a full collection of clothing racks and clothing rack supplies
Clothes racks are widely available at many department stores as well as online. These can be purchased at stores such as Walmart, Target, Sears, The Bay, Ikea and Bed, Bath and Beyond. They can also be found online on Amazon and eBay.