Maybe you can ask Proworld for a couple of samples. Don't know if they'll do that, but it doesn't hurt to ask.
I've received some in the past from other transfer suppliers. In fact, if you go to one of the trade shows you'll get several. They'll also send them every few months.
We just use what ever. Haven't had any problems with it.
The best thing to do is to convert to a Sublimation Ink, and Paper. We just haven't done that cause it is more expensive.
My wife had bought several cartridges, and I don't know where she got them from. Every now and then I have a big printing project, and I have several cartriges to use. One day I'll run out, lol.