Being that spring is approaching many regular home furniture stores will carry them. Otherwise contact places that carry beach and pool furniture. Often they are used for this as well

Most outdoor furniture stores will sell them, you could also try big department stores, and even hardware stores sometimes stock them. Other than that, you can always try ebay for a cheaper option, although if you want them quickly, keep in mind the delivery time.

Wal Mart, KMart, Target...

If you want some made especially for horse shows, I'm sure that Dover, or Back in the Saddle will have some.