Another vote for PRI pads. They come in every color under the sun and hold up really well. I have several for schooling and they just get thrown in the washing machine and hung on a fence to dry. Can't beat them at about $30 each!
I'll throw in another vote for the TuffRider pads. Mine fit fine under both my 18" dressage saddle and my jumping saddle. They're a bit big on my Arab, but at $15 I don't care. I also really like Roma's pads, but they're a bit more pricy at $40. The TuffRider ones are apparently indestructible. I still ride in the original one I got for my Arab 5 years ago. I've done endurance rides in them. I've done trail rides. I ride in them daily. The oldest one is a bit worn and has a few stains, but given what I've put it through, I've certainly gotten my money's worth out of it.
I'm thoroughly enjoying all the enabling, although my bank account won't I found some great colored ones on eBay that are reasonably priced, and have been considering a darkish teal pad for my mahogany bay. I also agree with the PRI recommendations: I've had good luck with their pads.