Last time i shopped for an AC Delco Opti Summit Racing had the best price. The AC Delco label was hard to find on the box at first but it really was an AC Delco unit. My last off-brand opti lasted <2 yrs (i think it was a Cardone unit but no longer certian). Make shure u order the late model ('96) Opti with the vacuum vent system (the first model Opti's had only holes venting to the bottom/atmoshpere).
Ok so i take a look at Summit Racing and see they want 450 bucks for an AC Delco now. Whew! I thought mine was like $250ish.
Good luck, sorry for the bad news,