Cafepress is a good place to order personalised t-shirts from. They have tons of gymnastics themes that can be put on anything! If you want better prices gymsupply.com has gymnastics t-shirts at decent prices.
Sami's HS team got shirts last year on the 40th anniversary of the gymnastics team at their HS--they said "40 years of being flippin awesome!"
Of course they're not allowed to WEAR them to school as the athletic director would apparently flip out about it (as flippin would be too close to another f word). But I think they're cute