I would look up a print shop rather than using one of the major websites. I loved working with a print shop that I could discuss the design with and get advice and production ideas; how to print gradients well, and so on. Often you’ll end up paying around the same, and can specify the t-shirt and exact printing method you’re looking for. Just look up t-shirt printers on your location’s equivalent of yell.com, call a few and describe what you’re wanting to do and get an idea of price. Then pay to have a test print done (if they can’t do that, which is quite common, then pay for a garment sample and ask for some photos of a similar print they’ve done). Note: You might need to order 30+ tees to start getting a reasonable fee. Good luck!