You can find some pretty good deals on Aliexpress. I would suggest you speak with different contacts in order to see which option fits your needs best. Quality is something that we are all looking for. It doesn't matter if we are offering or buying something. I've wanted to buy some Bitcoin Hats and Tshirts for my friends since they are obsessed with these things and I've managed to find some pretty sweet deals. I would rather pay more for an item and make sure that it will last longer than getting something cheap that is going to break next month
Amazon has come to a good supplier. Make sure that you are reading many reviews about different companies and the most important thing is to test the material quality. There is this method called heat transfer vinyl wholesale where you are able to make some quality prints and I am sure that it's worth checking out. I would also speak with my friends to see which material they prefer more since the customer opinion is more important than ours. That's the advice that my friends who are running different businesses have told me.