You could get a 18slot quiver with 15% haste from doing the hunter epic quest for the bow and staff.
With BC you can buy a 18slot
quiver and
ammo pouch with 15% haste from Kurenai and Consortium (you'll need honored faction).
With 2.3 there will be new pouches and quivers available. These can be crafted by leatherworkers and will have 20 (
quiver,
ammo pouch) and 24 (
quiver,
ammo pouch) slots.
The 24slot version will require a primal nether, other than that, the materials to craft these things are reasonable (perhaps except for the 99 light feathers for the 24slot quiver).