You can certainly google iron on or heat resistant vinyl and find resources as well as those that have been listed here
viewtopic.php?f=44&t=4814&p=86713&hilit=+places+to+purchase+vinyl#p86713If you want to buy large quantities of vinyl, many use US Cutters or Sign Warehouse for regular vinyl.
The craft stores now carry sheets of vinyl - like JoAnns, Michaels. even WalMart! If you just want sheets to be sure you will use it, vinylsuperstore.com has a good quality and selection.
Hope this helps you start discovering all that is out there -