You don't have to limit yourself to 8800GTS 512's, both the 9800GTX and GTX+ have the same number of shaders as the GTS and might be easier to find...
NCIX.com - Buy EVGA E-GEFORCE 9800GTX+ SC 756MHZ 512MB 2.24GHZ 256BIT DDR3 Dual DVI-I HDCP HDTV Out Video Card - 512-P3-N884-AR In Canada.,.. $189.99 after $20 MIR.