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Here is a list of Tshirt/Apparel companies that offer polyester or polyester blends. Some may only sell to businesses and require a TAX ID. I noted my favorite, but I’m sure you will have your own as well! My favorites were picked off of how often I used them, prices, customer service and most importantly, having a warehouse within one day transit to me! That means I can order by 5pm on Thursday and they arrive by Friday afternoon. Not much can top Amazon Prime, but they are killing it! If you are looking for a list of non-apparel vendors, check this list out here.
DISCLAIMER: I have not ordered from all of theses sites.
Sublimation Apparel Vendors
https://www.bellacanvas.com/
Heritage Sportswear (My favorite)
SanMar(Another Fav)