ps90 upper for ar15 lower: You're asking about the Rhineland Arms AR-57, which allows the AR-15 to accept PS-90 magazines, and eject the spent brass through the magazine well. Rhineland Arms seems to sell these exclusively through Military Gun Supply. According to their webpage for the AR-57, it seems that there is a large waiting list for these uppers now.
clips for a jld ptr 91 assault rifle: PTR-91's accept G3 magazines.
why cant you buy ammo in california if youre from out of state?: Some cities have local ordinances which mandate a California driver's license for the purchase of ammo. This is absolutely not a state-wide requirement, however.