Most bandsets are graded based on steel ammo. Ammo in the same weight range is fine to use as well.
Here are some helpful basics for selecting the correct bands for glass marbles or lead round balls.
For 12 - 15mm glass marbles, choose the General-Purpose Flat Bands 3/8” or Standard Flat Bands 7/16” size bands.
For .32 - .36 caliber lead balls, choose the General-Purpose Flat Bands 3/8” or Standard Flat Bands 7/16” size bands.
For .38 - .45 caliber lead balls, choose the Nitro Flat Bands 1/2” size bands.
All these bands can be found here:
https://simple-shot.com/accessories/2s1000xx/slingshot-flatband-bandsets