If you are targeting a niche audience, online can be good. But if you are targeting a wider audience and want to compete with existing retail brands, then online can be difficult to compete for exposure and sales. People like the shopping experience, trying stuff on, seeing and feeling the shirt in person, etc. So selling through stores would be better for that.
As for type of stores, again it depends on the brand. Try finding stores that carry similar products at similar price points and contact those owners or buyers.
If your market is a real niche market then it's usually better on the internet as people will find you better. But if what you are doing will have a mass appeal then try to get in the stores (and have a website too).
I'd do both if possible. It's much easier to get noticed and be taken seriously if you're in stores or otherwise selling locally. You can use the exposure from selling locally to generate traffic to your website. I'd look for stores that carry similar products and/or sell things that appeal to your target market.
Even if you are in a niche market it will still take a lot of time and effort to generate traffic to a website, and in general it is much harder to sell something online, since customers only have pictures to look at and they can't see it in person/feel it/try it on etc. However, having a website is a good idea since it can help generate sales from people who don't live in your area.
use any means necessary. have all your friends wear your product and you can get exposure that way as well. When someone asks who did that you can have business cards made up to show them how to find your work. Whether it be online or in a store.
Who are your target customers and where do they make purchases? Those are the places where you want to sell your shirts. The "best" place to make a sell will vary based on how you answer those two questions.
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