I have tried different pouches for holding slingshot ammo on my belt. You want something you can close up tightly, so nothing gets in or out, but when open it does not sag closed (so you can get your hand in there easily). Ideally, you want a flap that will cover the opening that does not get in the way of your hand going in, but still prevents the ammo from bouncing out when the pouch in is the "open" position.

You can pay upwards of $20 for something like this, that still doesn't work perfectly. Or, you can spend $2 on a surplus "Swiss Oilcan Pouch" from WWII. After I washed the war smell and grunge off the canvas pouch, it filled my needs perfectly. (The expensive alternatives for such a slingshot ammo pouch are small rock climbing chalk bags or doggie treat bags.) I'm going to order more of these WWII pouches. The are useful for holding many things easily accessible on your belt. Maybe a bunch of loose .22LR ammo for a hike/shoot in the woods - things like that.

http://www.gunpartscorp.com/catalog/Detail.aspx?pid=102920&filter=102920&catid=0