Sell crypto shirts for ICO's, free marketing plus you get sales.
This is what's the idea on my mind that you can ask them to order t-shirts to you and will use it to reward their investors who allocated and contributed big amount of money to their ICO. Its not a bad reward for them and for sure many will buy on this idea. Profit + marketing, its a win-win situation for you and also to those ICO's who will be interested with t-shirt ideas.
This is only a way where you do only distribute your t-shirts not actually starting up on the business and it seems op wont really start up to have its own ICO just to sell out shirts. Win-win situation but isn't a guaranteed that those shirts would really get attention or popularity. Starting up on your own brand would really require first is the concept/idea of design since this thing would really matter the most in the beginning.