Don’t discount loopnet right away. It’s true that most of those deals have been picked over, but through analyzing them til you can’t see straight, you’ll gain a quick way to really understand your market better and get a feel for all the trends and averages. Those are very important to know.
As for letters, I’ve found those can work for smaller deals, but for most mid to large size deals, there has been less success. Letters sound better in theory than in practice…it’s more like panning for gold…sometimes you get a nugget or two, but most often it’s full of dirt.