I'm new to this board and also live in Mass. Aside from the Deerfield River, you have some trout water much closer to Medford. There's the Nissitissit River in Pepperell and the Squannacook River in Townsend. In Pepperell, there is a fly shop called the Evening Sun Evening Sun Fly Shop - northern Massachusetts and southern New Hampshire - Fly fishing & fly tying supplies, equipment, and instruction
that can give you the lowdown on those 2 rivers. Further west, there is the Millers River on out Rt 2 and the Swift River which flows out from the southern end of the Quabbin Res. There are also lots of smaller streams close to you that are stocked every spring. There is the Bass Pro Shop in Foxboro, right by the stadium, that has a decent fly fishing section. There is also a LL Bean in Mansfield that I wouldn't recommend. If you were to travel to Mansfield, you would be better off to take an extra 15 minutes to go to the Bears Den Fly Shop in Taunton Bear's Den Fly Fishing Co.
. There is saltwater FF nearby, but that's another ballgame.