Check out EMCEE apparel, they have a huge variety in colourful shirts that are very high quality material and breathable. They are around $100 (I think) but are well worth the money. You could also try QJ Riding Wear or Ariat.
There are so many brands of show shirts out there! And no, you don't have to spend a lot of money to get a nice one. I really like the kind Noble Outfitters carry. They are supper nice quality, well ventilated, and look flattering on. I recommend you taking a look :)