Unfortunately, not all carts have them on. Sometimes they need to be ordered seperately. I needed to order them for my mini show cart a couple of years ago. You would think that with what they charge for some of the carts--the carts could come with stops, but they cost me something like $14 or $15 extra.
Smuckers Harness Co and Ozark Mtn Mini have them in their catalogs, in pony size, along w/ other online stores. Seems like they are an option w/ most cart makers but it wasn't offered at the time I ordered my cart 18 months ago.