#1 Shot is .30 caliber, and #0 is .32 caliber. They can be bought in quantity for much less. See Ballistics products, among others. Hornady sells buck shot in bags, and I believe, bottles.
Here is the sizing on Buck shot for everyone else:
#4 Buck: .24
#3 Buck: .25
#2 Buck: .27
#1 Buck: .30
#0 Buck: .32
#00 Buck .33
#000 Buck .36
To determine the diameter of bird shot, use the Rule of 17: Subtract the # of the shot, from the number 17, and you have the diameter of the pellet in hundredths of an inch.
Steel BB are .18( as opposed to the BBs you buy for a air gun, which are .177" That is why we don't use the steel BBs sold for shotguns in these air rifles, the pellets stick in the barrels and ruin them! )
T-steel pellets are .20
F-steel pellets are .22
Hope this helps you. The only way you could possibly save any real money casting a .31 size ball is you had a gang mold like you see sold to fishermen who want to cast their own split shot for line weights. That little ball only weighs 40 grains or so. Do what other shooters do. They buy the buckshot in bulk, and just have fun shooting.