Drilling glass is tricky stuff. It can't be done on tempered material, which is used on most tank bottom panes.
The average lfs
just sells predrilled tanks - they don't have the equipment or skilled staff to do custom work. I don't think auto glass shops would be of much help either.
I would check in the local yellow pages under "aquarium maintenance" or "aquarium manufacturers". Someone out there may be willing and able to do the job, or at least steer you toward someone who can. Good luck.