I didn't quite understood your question, do you already have a plenum that accepts additional injectors
but you can't find suitable injectors or you are looking for a complete system to add on your bike?
In any case i do have secondaries fitted on mine but if i was going to do it all over again (on a gen1 and up to 300hp) i would swap the 4 stock injectors with 4 high impedance 550 cc injectors (ask which ones because i don't remember which ones give good idle) and a power commander Pti.
If you definately want secondaries then you must buy a plenum that supports additional injectors (very expensive), at least 450 cc injectors and a secondary injector controller (i have the AIC from split second).
Chris