Normally most bands say what Ammo they are rated for, tbh unless you are hunting the smaller cheaper stuff will do, even pebbles will do as thats what I used learning as a kid, a Catty isn't fussed about ammo like a Rifle is so the cheapest bag or bottle of it will do.
I've been using some old Modelling clay to make my own that I keep in Mint tins, uniform as I can so I can keep the same reference points but if you've got the means you can recycle old Pellets into Catty ammo.
You'll Shoot your eye out Kid
Currently looking for an SMK/BAM B4-4 Rear sight - Possible Candidate found, await delivery to check.