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.