Alliance - Stormwind. Near the test dummies there is a room with a bunch of vendors.
Horde - Orgrimmar. Hall of legends (Across rope bridge from flight master).

75 k honour will get you about 2 pieces. Shoulders and gloves gost 31600 honour a piece. Other gear such as helm cost about 49k.

You can buy the pieces before 80. I bought two pieces when I was 77(obviously can't wear them) but it allowed me to accumulate more honour towards a third piece before I hit 80.