For single (not bonded) wire check out
www.tinnedmarinewire.com. They have quality tinned marine wire for less money than lesser wire from other sites.
If you need 6 AWG wire for battery cable, I suggest moving up to "welding cable" with EPDM insulation. It is much more flexible than regular battery cable. Cheapest supplier is
www.remybattery.com, where you can order it by the foot.
While you are at the remybattery site, check out their battery terminal covers. The "starter terminal" cover or the "straight or angled" covers beat the heck out using electrical tape.