I didn't get speakers with built-in amplifiers, but I got some 6.5" speakers to fit in the slot and just taped the existing amplifiers to the speakers (has been holding for a good 2 years now
My initial problem was that my front right speaker started popping for no reason (I figure if I had been listening to music that loud all my speakers would have popped, but it was just that one). Anyway, I ordered some Pioneer speakers as I've had good luck with them in the past and they're doing great.
I got 3 way speakers (I have dual 12" subs in my cargo area, so I needed the mid range) Pioneer USA - TS-A1673R - 6-1/2" 3-Way Speaker with 180 Watts Maximum Power
are some that would work.
Basically I just had to unscrew the amplifier units from the stock infinity speakers and just hook the wires up to the new speakers (and attach them together, tape or epoxy would work fine).