some of the supplies like Bodek and Rhodes have downloads available of there products that you can import into corel and lay your designs over them and print them out for your customers. You just need to set up a account with them to download there stuff. check with you local supplier to see if they have anything like that you need first.
There are free resources available all around. Do a search on Vector T-shirt template and you'll get some great results.
Can't remember if I made this or it was downloaded...LOL.