contact the local high school Athletic Directors office and ask where they buy them. Typically there are distributors in every area than can save you money when you are buying in bulk. I've seen pricing anywhere from $36 per dozen to $78 per dozen on the same exact balls so shop around a little. If you can find pricing around $50 a dozen that's pretty good for good quality balls.
I have bought from them online in the past, usually delivered in two days on the east coast. They have a pretty good selection of 11" optic yellow. We use a "Dixie Softball" #10 "safety" core for our 8U travel and in town rec. It is not a mush ball, but is has a slightly softer feel.
I've bought new 11 and 12 in balls on E-bay for around $2.50 each in the past couple years. Good balls, Worth red dots, etc. You have to look around but they are there. Also practice balls you can sometimes get from a 1st year 12U team that has moved up to the 12in ball.