I use 0.40 or 0.45 bands to shoot 7mm steel or 10mm clay ammos.
Very light pull and great projectile velocity.

I bought these bands from Amazon when I was at the beginning and they were pretty decent.

Once you know what you like to shoot, the cheapest option is to buy a roll of latex, some pouches, and all the stuf you need to DIY, from one of the known Chinese sellers ( Snipersling, GZK, Slingshooting.com)