Not sure if it meets your geographical criteria but Bass Pro in Foxboro ususally has some. haven't been in there in a month so you should let your fingers do the walkin before heading in that direction
I hear Flint Armament in Fall River is carrying some powder and also Village Sports in Westport.
Also, I'm not really a big fan of either of these stores two stores but Northeast Trading in N Attleboro and Middleboro Gun Shop usually have a good selection of powder.
finding Titegroup is like looking for bigfoot...heard its out there but haven't found it yet. I was in Bass Pro the other day and didn't see any and I don't think flint carries a lot of powder. NET usually has a lot of powder if you dare go there.
Not south shore but, I recall seeing it on the shelf when I bought powder and primers at Collector's Gallery in Stoneham a couple weeks ago. Give them a call if you can't find it elsewhere.