For local shows a nice shirt that compliments your blanket choice with dark jeans and boots is very acceptable. I do ranch horse classes so I don't have to go crazy, I find most of my shirts on Ebay. You can get name brand shirts for good prices rather than buying in a western store.
I think Beau159 on here does some show/rodeo shirts for a good price too, if you want something with crystals.
Or if money isn't an issue you can go all out on custom made show shirts!