If you cannot find your property for in the Suffolk County Registry of Deeds site under regular deeds, Look in the certified land section. If on line searches fail go the the court house on New Chardon Street near Haymarket. The staff will direct you to the place to find your plot plan.
You may need to check under "plans" instead of "documents" at the registry's site. I just grabbed some for an FHA condominum approval I am doing under FHA's new system and found the plat plans but it took a little poking around.
Heather, It should be recorded at the Suffolk Registry of Deeds. It should be with the Master Plans. Also which would be far easier go on to the City of Boston Assessor's website. Plug in the address and the page that will come up will say details or maps. The map you will see will not be the actual official plot plan but you will get the idea of the lot lines.
Hope this helps.
Jessie Cuddy, Realtor
Boston Bayside Properties
2253a Dorchester Avenue
Dorchester, MA 02124 http://www.jessiecuddy.com