We got ours from Menards...but not pre-drilled. DH drilled the holes and we have them at either end of the tub in our bathroom. When we remodeled the bathroom, the we put in a big soaker, jetted tub. It is a foot longer than a standard tub. To get glass shower doors would have cost $1500.00!!!! so we installed a stack of 6" blocks at either end, (with blue led lights inside)...voila..standard shower doors will now fit.
Good luck finding pre-drilled. Let us know if/where you find some.