Here are a few options for you. I didn't check to see specifically how close to Boston these all are but they're all at least in MA. A call may give you suggestions or ideas for other outlets from those that are a little far away. You would certainly have lots of testing grounds from these and possibly some good "buying sources" as well.
Phoenix
http://www.goodnightnaturals.com/ Retailer. Has a good assortment of different latex mattresses. Their names are a private label but could be WJ Southard mattresses and/or others.
http://www.gardnermattress.com/products.asp Manufacturer. Has a range of latex options including layering. Good feedback here and elsewhere abou their quality.
http://www.bostonbed.com/products/bbc_mattresses.html#pll Retailer. Factory direct for Therapedic (national manufacturer for Kathy Ireland and others). Has some "mostly latex" and some "some latex" options but probably least valuable for latex testing here.
http://www.flybynight.com/Natural-Mattresses_c_8.html Retailer. Carries OMI (premium latex), Savvy rest (layered zip cover do it yourself latex but fairly expensive) and Berkeley ergonomics (some interesting choices).
http://www.furnature.com/furnature-organic-serenity-mattress.html Retailer. Carries Savvy rest layered do it yourself as above
http://www.jordans.com/Mattresses.aspx Manufacturer/Retailer. Makes their own brand of partial latex options as well as carries Natura (and others) as above
http://www.theorganicmattress.com/mattresses.php Retailer. Carries a wide range of latex including layered do it yourself (possibly Lifekind a sister company to OMI)
http://www.ecinindustries.com/products.htm Manufacturer. They mainly supply institutional and private label to retail outlets. A call would discover if they are supplying to any local outlets near you.
http://www.mysticbedding.com/ Local manufacturer. Mostly innerspring (incl 12.5 gauge). Included for reference as I didn't see any latex options on their site
http://worleybeds.com/ Local manufacturer. Seems to make a wide range of mattressess including latex options but no specific mattresses on their site.
http://www.reverie.com/about.htm National manufacturer of adjustable beds and latex mattresses. I included them as it would be interesting if their latex mattress was available locally and if you had a chance to try it (for our sake). Another thread is talking about what their latex really is as it's likely from China using a different method of producing latex than either Talalay or Dunlop.
This message was modified Dec 7, 2010 by Phoenix