A Digital Printing Network
A product like Imprintables Solutions Opague should work for your flag.
If in doubt, I would always use something that will work on nylon, like Butch said.
Stuff made for nylon will work on polyester
However stuff only made for cotton or poly won't work on nylon
Better to be safe.
Colorprint Premium from Imprintables would probably do the job. I use it on nylon and poly bags.