18G to draw with,
and 23g or 25g depending on where you are shooting and how much padding you have your ass cheek.
I would also not say that they are readily available at Walmarts or pharmacies.
States stay you can buy them there, but when you go to get them, they will determine whether you need them or not and more often than not they won't sell them to you.
You are better off using AndroUsa, Gearboxinc, etc.