I use Phoenix Gold TI-122SB, 12 guage, very thick PVC outerlayer, 2 conductors, twisted, each conductor has 7, 8 bundles, each bundle has may be 20 strands (I didn't actually count, all estimates). I only found 250 ft spool for about USD 135 (plus 16 for S&H).
http://shop.store.yahoo.com/lpcs1/elbout-new-arrivals-phoenix-gold.html
http://www.efunctional.com/electronics-new-phoenix-gold-cables-and-connectors-speaker-wire.html
Here is a retailer who sells by foot
http://www.sounddomain.com/shop/list/b/Phoenix%20Gold/t/Speaker%20Wires
Phoenix Gold has two other 12 guage cables, non-twisted, in 50 ft spool.
M-1250 in the $23 - $25 range
http://www.123av.com/Store/Cables/047.asp
http://www.vss-electronics.com/m-1250.html
QS-1250 in the $20 range
http://www.123av.com/Store/Cables/047.asp
I also read positive reviews on Carol Brand 12 gauge sold by Home Depot or Parts Express
http://www.partsexpress.com/webpage.cfm?DID=7&WebPage_ID=3&Start=2&searchorderby=1&Manufacturer=33&so=1
