I would suggest to look into a wand massager like the LELO smart wand. It's rechargeable, cordless, waterproof and somewhat expensive. It is a luxury toy though. You can also use it as a clitoral vibrator if you want! I have both and use mine for back massages all the time.
Smart wand Medium
Smart wand Large
Other ideas can be found by just searching for 'wand massager', or looking here