Go through AAFES to Brigade Quartermasters (actiongear.com for every one else) for a military discount.

Look in Home > Tactical - Vests, Holsters, Rappelling, Ropes > Tactical Load Bearing Assault Vests & Gear Pouches.

For a BOB aplication, Camelbak's DELTA-5 would be a good option as it already carries 102 oz of water for you.