Hi Ryan!
There's a guy here in Australia that started the Attitude brand. Apparently for a few weeks he got all of his friends to ring up the shops he wanted to be in, and ask them if they stocked Attitude gear. Obviously they didn't at that stage, but each time a new friend called, the shop owners began to think, "Wow, this is a really popular brand!"
Eventually, he would call up himself and say that was the owner of the Attitude brand. Then he'd ask if they would like to sell his stuff. Of course, they were more than happy to!
This guy ended up building the brand and eventually sold it for $5 million or something crazy.
You may or may not like to try that...
Many store owners will allow you to sell your t-shirts on consignment. This is probably the easiest way to get in, as it is no risk for them. Make sure that you get their commission arrangement in writing (they'll generally take 50% of the sale and give the rest to you.)
All the best!