There is, and have been, a good selection of Maine 2-footer railroad structure kits, and SR&RL kits in particular. Unfortunately, many of them are long gone and out of production. Fortunately, many of them are still in current production, and many of the old ones show up on Ebay from time to time.
Farmington Water Crane
The Farmington water crane kit was produced by Sandy River Car Shops and is out of production. I have picked up two of them off Ebay over the last couple of years, and have seen more of them show up on Ebay. I have built two, with one closed in and the stand pipe and valve wheel repositioned to resemble the water crane at Phillips.
Ball Signal Kit
This is a B.E.S.T kit, which Stands for Bollinger Edgerly Scale Trains. The kit is a replica of the ball signal at Whitefield, New Hampshire, but will stand in admirably for the ball signals used on the SR&RL. If you want to make it more accurate for the SR&RL, modifications are pretty straight-forward. The kit comes with two complete ball signals, so you can equip two yards, but if you want to model the triple-ball signal at Strong, you'll either have to steal one of the balls from the second signal, or buy an additional ball, which are available as individual castings from B.E.S.T.
Freight House at Strong
The freight house kit is produced by New England Scene. It makes a very handsome kit, and is so typical of Maine freight sheds you could use it just about anywhere you choose, with no complaints. It uses the common 'peel and stick' system for windows and doors, which I don't particularly care for, but hey, I'm grateful for every kit for the SR&RL that anyone produces.
This is an American Model Builders "LASERKIT". Again, the peel and stick system is used, and if you build this one, beware you'll have problems with the bay window area at the office end of the depot. It's not just me, others have had the same problems. Be prepared for the wooden panels there to break at the window holes, the wooden components there to not line up and fit on the floor base, the peel and stick windows not to line up and stay stuck, etc. Just do the best you can, and once you have it in place on the layout....leave it there and don't touch it.
New England Creamery
Unfortunately, I appear to not have photographed or saved the box for this - update, found the box end, photo below, it was a Sandy River Car Shops kit. The kit didn't come with materials to make the foundation, or decals/transfers. So I made a foundation out of styrene, and used rub-on transfer letters from the craft shop to add on the lettering. I don't know what it is about this simple building, but I love it. And if you ever get up to Strong, it's still there where it's always been, looking pretty much the way it always did.
Engine House at Strong
I didn't photo or save the box to this one either, so I don't recall who makes it, - update, found the end of the box, it's a New England Scene kit. Another peel and stick kit. The hinges for the bay doors are laser cut paper and are peel and stick. Be prepared for the doors to fall off at some point, and then reattach the hinge plates to the building with CAA.
Kennebunk Models Kingfield Covered Station
Saw on Ebay
Kennebunk Models SR&RL Freight House
Saw on Ebay.
Kennebunk Models Sanders' Station
Saw on Ebay
Kennebunk Models SR&RL Section House
Saw on Ebay
APAG Hobbies Turntable kit
Sequoia Scale Models turntable kit
Haydenton Covered Depot
SThis is an out of production Quality Craft Models craftsman kit. This techinically isn't a Sandy River RR kit, but it's based on the covered depots that were built on the Sandy River RR, and it would take aficionado expertise to know the difference. So I include it here. If you look at the photos of the first depots built at Phillips and Strong, and want to model those without scratch building them, this kit is for you. If you don't get the Haydenton name, it's named for Bob Hayden, an early pioneer and HOn30 model icon in Maine model railroading in this scale. His partner, Dave Frary the other icon, is the object of the Fraryville depot signs on the sign sheet. These turn up on Ebay every once in a while. Someone got theirs for several dollars less than I got mine a couple of months later. Salud to you!
Portland Locomotive Works Kits
Kingfield Roundhouse kit
When they talk about 'craftsman kits', they're talking about kits like those turned out by PLM. They are pricey, but when you get these in the mail, and think about what goes into designing and producing a limited run of kit for a niche market, you should understand why when you start pulling stuff out of the box. There are no instructions included with the kits. It helps keep costs down. They are available online through PLM on the "Box" files website. You can print them up there if you want them on paper.