Where is the best selection of costumes for the stage?

I think you should be more specific about which country you're interested in shopping for. If we're talking about online stores, I think everything is already available on Amazon.

Recently, you can find a lot of offers on the Internet, and for all tastes and budgets. There are inexpensive as well as amazingly gorgeous costumes. Look here https://www.etereshop.com/ offers, they seem to me to have the best value for money