Try Belden in bulk; good insulation and available in 12g. You may wish to review this McIntosh Lab test report of speaker wire gauge and length and performance results.
There is a table showing the recommended gauge for specific lengths of wire runs at this site.
http://www.roger-russell.com/wire/wire.htm
There is a table showing the recommended gauge for specific lengths of wire runs at this site.
http://www.roger-russell.com/wire/wire.htm