Reserva Privada is a branch of DNA Genetics, which, like so many other breeders, is based in the Netherlands. I'm not privy to the exact growing/sales arrangement, but I believe the seeds are actually produced in Holland.
So yes, this is basically a case of exactly what you're describing, namely USA-bred lines being grown for seed in Holland, ultimately for commercial sale.
I don't know where TGA/Subcool makes the seeds, though I think Subcool sometimes posts on this board, so you could probably ask him directly. I strongly doubt its in the USA, though. My guess, again, is probably Europe. The website also lists a bunch of sites in the USA where his seeds are available, so I imagine the same thing holds true. The strain development and commercial seed production may well be on different continents!
As to an open seedbank in the USA, I wouldn't hold my breath for that. Possession of seeds is still illegal in all 50 States under Federal law.
The Obama administration started off tolerant of medical marijuana, but its not been so much lately, and I don't think it would tolerate a seedbank moving product all over the country openly.
I know that you can sometimes find seeds for sale at dispensaries, and again, TGA does list seed distributors in legal MMJ sites on its website. So if you live in a legal MMJ state, you can probably find a distributor that will sell you seeds.
Obviously, you can also probably find seeds on the black market, or find other individuals willing to trade for seeds, though these are obviously not the same thing as a seedbank with a huge variety of product selling on the open market.