Hi Virginia
One way is to ask the manufacturer to source it for you. Manufacturers most of the time have great relationships with suppliers and can as well get good rates. They might even have this service in house. Also the manufacturer has your material and can easily test directly on the material to make sue the quality is good. If they don’t have this service in house you can ask them for recommendations to a heat transfer supplier.
Here are some other options for you:
http://unimarkusa.com
http://laurema.lt
https://www.regineiqtrim.com
https://www.safereflections.com
http://i-sealon.com/en/
https://www.barta.at/en/
http://www.seripress.com
https://rudholmgroup.com
🙂