Welcome to the forums, Ana. Just some ideas here:
1) There is a hybrid paper for sublimation on dark & cotton fabrics. This might be a good time to try it. iccink.com sells it in the States,
International Coatings
2) You can try the print&cut media being sold by imprintables.com. Josh is a member here. If you don't have a print&cut machine, I think they can also do it for you.
3) If the volume qty order permits it, you can also order custom made plastisol transfers.
4) You can also outsource the job to your local screen printer (again if volume qty is not a problem)
5) Some opaque transfers for laser or inkjet could also do the job, but the quality you'll get is somewhat inferior.