Apparently, the combat rifle with mods and marksman perk is a beast, and I've wanted to try a sniping setup. How do I get a combat rifle early on?
From the Combat Rifle wikia page, you are able to find a Combat Rifle in the following places:
Can be purchased at the Diamond City market.
One can be found next to a dead soldier next to a beached tank directly west of Libertalia.
One will spawn at any level on top of the roof of the Greater Mass blood clinic, next to the body of a dead survivor.
One can be found during the miscellaneous quest Find the Treasures of Jamaica Plain.
On the east side of Greater Mass blood clinic in the back of a burned out vehicle there is a chance one will spawn next to a skeletal soldier.
In general, you will be able to find them as drops from enemies, starting around level 15, though.
Around level 15, Gunners will start equipping these with large magazines and long barrels.
Fairly common among raiders after a certain level
At higher levels, the combat rifle and modded variants can be found on almost all the mercenaries at Parsons State Insane Asylum along with plenty of combat armor. These respawn almost daily, making them ideal for obtaining many for settlers.
One potential spawn is on top of the wrecked building on one of the small elevated parts of the floor where you find the BoS patrol in the quest The Lost Patrol, you will need to jump on a file cabinet to reach. Note - this spawn may be levelled, at lower levels a pipe rifle has been seen.
Based on the available information, I would say your best bet is to go to Libertalia or do the Find the Treasures of Jamaica Plain as those are both guaranteed Combat Rifle locations. After rereading your question, I realized you asked for northern areas specifically. Libertalia is slightly further north than Jamaica Plains, but it is further East. I do not think either of these should be too difficult for you to clear out and get a Combat Rifle from.
Note: I would not recommend buying one from Diamond City market at a low level as Combat Rifles tend to be quite pricy, especially for a low level character.
There's one on the road from Covenant to the Medford Memorial Hospital, where the caravan was hit in the Covenant quest. (Don't need to start the quest) You can get this rifle AND the .308 sniper rifle from the Red Rocket between College Square and Mass Sand and Gravel at level 2. You don't even need to be very sneaky.