Blitz buys injectors from NipponDenso. There is no other way to buy a denso in the states that I have found besides either Blitz or via a dealership that sold them originally (IE Mazda).
There are two different Blitz 850 part numbers. One (the cheaper one) is just the injector. The other is the injector (a Denso 2450), an adapter ring, and an electrical connector. Our cars need the more expensive kit.
These injectors came stock in FD (Gen III I think) RX7's and were used as the secondary injectors. They were installed as pairs on the FD's as the primaries are a different injector rated at 550 cc/min. If you find four FD secondaries, you have the same injector, but still need the adapters. I can sell the adapters to you for $30 each custom made by us! With the adapters we sell, and a bit of work with an exacty knife to make the injector compatible with our stock electrical clips, you can have a nice set of 850's that will fit the WolfKatz Rail and the Gen III Rail.