Harford County Maryland is probably the closest to you that you'll find it.
Southern NJ/Northern & Central DE/Southeastern PA is Canada Dry Delaware Valley. They don't produce Squirt.
Further west and north in Pennsylvania is Davis Beverages. I'm almost certain they don't produce it, either.
Canada Dry Potomac does produce it, though. But only in 20oz 6pks.
Travel down I-95, pass through Delaware, and then go over the Susquehanna River. That'll put you in Harford County. There are plenty of places to find it, right off the highway.
Exit #85 -- take 22 East towards Aberdeen. At the second light, you can make a left to get into the Klein's Supermarket shopping center, or a right to get into the Mars Supermarket shopping center. One or both of them should have it, and Klein's often puts Canada Dry 6pks on sale. They also have Welch's Peach and Strawberry, which are pretty rare flavors around here in 20oz form. The various Diet Rite flavors are also showing up a lot more as well.
Thanks for correcting me on Davis. I didn't remember seeing Squirt out this far from them (they have pretty good representation at the Super Wally in Parkesburg, PA).
It's also a little more difficult to find all the flavors of 16.9oz from them. I only remember seeing PA Dutch in Sunbury. CD Delaware Valley imports 7up, Sunkist, A&W, and CD (maybe diet, too) in 16.9 from Davis.