Silk screening and screen printing can be costly.
But if you are looking to do a simple one color print, I would suggest going to the local craft store. I know Hobby Lobby has an at home screen printing kit.
Online clothing store for used or new clothes?
I know people have their online clothing business where they sell used, vintage clothing. You can find some amazing vintage used clothing etc. at your look thrift stores at incredible prices and sell those to your online audience.
If you are looking for new clothes then you can try eBay wholesale lots. It depends what kind of clothes you want to sell.