Swampy,
I don't know where you are in Southern New England, but it is a pretty small place
Whole Tone Guitars in Glastonbury, CT always has a good selection of acoustics in stock, including lots of Martins. Nice shop.
Martel Music in Plainville, CT has fewer but they do seem to come across some interesting used pieces.
I've not been to Acoustic Music in Guilford, CT but I hear very good things about them.
They all seem to list new and used inventory on their websites.
Happy hunting. My 2 cents, if I wanted a dread at your price point I'd be looking for a used D-35. I've played a few and they were wonderful.