If the speakers can only handle 90 watt rms why do you want 100?
I'm not sure of your setup or anything but the way I understand it you can wire two crossovers for a 2 ohm load but then you will only get one channel out of it (not stereo). Also most amps will make a lot more power and noise at a greater load, lower impendance.
I guess the operative word is cheap for finding a 100x2 amp. Online is the best place to go, at least for me. Markup in my area is pretty gross. That said you should try: www.wooferetc.com www.onlinecarstereo.com or if you're desperate www.crutchfield.com (i'm not even going to link that one)