Can't tell you where to but it as I buy all my cable (100's of thoudsands of feet) through authorized distributors like Anixter, Graybar, CSC, etc. Stay with a name brand and determine if you require plenum rated or non-plenum rated. Plenum rated is much more expensive but required by most locals when running cable trough plenum rated spaces.
BTW why did you want category 6, category 5e is great for up to 1000BaseT and perfectly fine for home LANs which typically run at only 100BaseT. You can get non-plenum Category 5e also in Home Depot at a great price.
Joseph Golan, RCDD