That's funny cos we have a ton of purple saddle pads in all our tack shops over here in Aus! Just come down for a visit
I would recommend looking online. There's a heap of horsey websites that let you buy online, and a lot of them that I have seen sell all sorts of saddle pads and other colourful accessories! I don't know anything about webistes that are based in the US, so I'd just start at google. Ebay
might be worth a look too!