i use a small canister that i found at an army surplus table at the gun show. it was a personal decontamination kit. hard plastic, with a somewhat softer plastic lid. long the lines of a film canister, but about the size of a closed fist. lid was tethered to the canister by a nylon loop with a small wire clip/clasp. you could thread your belt through the loop. held about 250 shells. i bought it for a buck, but rarely see them any more. if i see them again, i'll buy every one i they have.
another option, acecase.com has a .22 ammo holder, that is like the above mentioned elastic band butt ammo holders, but instead of elastic loops, there is a small pouch, with a snap top. probably holds 50-100 rounds, for around $5, if i remember correctly