The don't carry the replacement on fastech it just that most other tanks they sell are much cheaper there in the 1.50 to 3$ range. The best deal I could find for the Prometheus was at spectrum as a matter of fact it was the only place I found them at all. It looks like I may have had a bit of luck finding my Kraken 26650 tank replacement after writing to almost every one selling them on ebay I think I found a gut that will sell me 3 tanks for 12$ shipped
so after 2 weeks of looking I may have had a spot of good luck finally, If this flies I will be dancing LOL 4$ each shipped wow that is great if the deal goes down.
I would just snag them nice colored tanks from spectrum if I was you it a nice upgrade for that units you have. They came with plastic stock on the authentic Prometheus so even getting one from them would not be a great option. So scoop that nice custom glass tank and run. Like I said when mine come in I am going to grab a couple nice color tanks for mine too once it get here if it ever gets here as slow as Fastech ships the should have called that place slowtech LOL