Any website that carries the BR event collection might have them! BR has tons of colours in polos/saddle pads/bonnets/boots.
If you're in Canada, I know that Apple Saddlery carries the BR event collection.
Greenhawk (also Canada) carries Shedrow open front/hind boots in purple too! They're under $40 for a pair I think :)
Hope you find some!
Hi Rose! I think you can find purple jumping boots on Ebay !
Good luck on your research !