Borrow the battery from your (friend's, neighbor's, enemy's, etc) car..... Two batteris in series (maybe 2 sets in parallel) would run for a while. The other question I'd ask is how are you going to adjust the speed?
Try an industrial alarm company. Most emergency lighting systems are battery powered as well as alarm back up power supplies. These batteries must be replaced on a regular schedule even though they have plenty of life left in them. Most of these batteries are Gel Cell so mounting is easy. I've gotten some that have amp hour ratings higher than most motorcycle batteries. The best part is they are usually free when you can get them.
Worth a few phone calls...
I'm the monster under your bed....really!