eBay is usually the best place. You can get them brand new from Helm but you'll pay multiple hundreds of dollars.
There are three books you'll need:
Vehicle-specific repair manual
Unit repair manual
Electrical diagram manual
If you bargain shop on eBay, you shouldn't have to spend more than $20 each.
And yes... it's definitely worth it, if you plan to do ANY of your own work. Light years better than Chilton or Haynes.