I wouldn't think a spring weighing 100-150 pounds is what is needed for a buggy. Maybe that size would work on a Hitch wagon, but might be overwhelming on a light buggy. Did you actually put it on a scale?
Best place to find carriage springs is Witmers in Penn.
Phone is 717-656-3411.
Address is, Witmer's Coach Shop, 1070 W. Main St., New Holland, Pennsylvania 17557
I would call and talk to them, they offer a huge variety of carriage parts. Springs in every design and shape. Hope you find the spring you need.