Maybe a shop on Etsy. I've got an idea for a niche, but know very little about the t-shirt industry. I've got Etsy experience but want to find out as much as I can before investing.
Best bet is to contact a local contract printer, and show them your idea. Run a small batch of shirts, don't focus specifically on profitability but on building a market. Once you get that market, you can run larger batches and make profits.
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