Where can I find a cheap dressage shirt?

OK, so I have a horseback riding show in like, 2 weeks, and I have to dress properly. That means riding pants (have that), riding boots (have it too), helmet (got that), and a white button-down dress shirt, like a special shirt, and I don't have it. Where can I find one that's cheap? because everywhere I look, they are pretty expensive. Please help! Thank you!