I second the Kelty packs.

If you're looking to stay military (based on your OP), there are the "Bug Out" bags, aka 3-day bags, that are essentially daypacks. Also you can sometimes find "small" ALICE bags that are pseudo-military. Don't forget that there are day packs on the newer MOLLE packs, that are sometimes sold seperately.

Cheaperthandirt.com
actiongear.com

both have a good selection of military gear, though CTD tends to be knock-offs of originals.