Tennis players are TALL. Where do they get their gear?

I just watched John Isner and Reilly Opelka play at the Citi Open in Washington. Both men are around 6'11"--giant. I, too, am very tall. Where can I get a T-shirt in a size that would fit one of these guys? At Tennis Warehouse, when I look at the pros' gear and find the shirts they wear, it only offers sizing up to XXL. Surely these guys are wearing some other size, like XXL XX-Tall (if that exists). Do they just have their gear custom made, or is there a site somewhere that sells crazy tall...

Tennis players are TALL. Where do they get their gear?