That sounds like a tall order. If you are lucky enough to have access to a small mill, they might be able to custom mill it. The other thing is to call some floor sanding contractors. If they work on old houses, they would typically be called on to patch floors and might have some ideas. Even if you did find 3 1/2" flooring, be aware there is no guarantee that the tongues and grooves will match. I've run across that with run of the mill 2 1/4 maple flooring. Really tough if you are trying to weave boards in. Perhaps there is a closet or carpeted area you can rob some from?

Good luck on your project!

 

also be aware that any new pine you'd fine would likely be softer and not have as tight a grain compared to what was used 120 yrs ago.

 

google reclaimed pine flooring and then make some inquiries.

Here's one I found: http://www.southendreclaimed.com/pr...BRBNEiwAt0Znw2HM-dPpHSbqAy8CW9cFlZE1Tcgn0b5V1UJpkMcjf7j7Kp4kUHng9hoCl5kQAvD_BwE

Any competent millwork shop should be able to match the tongue and groove if you find oversized pieces.

 

The wood that floor is made out of is not available new, basically because the trees don't exist. As mentioned you need to find reclaimed boards or flooring and have it milled to what you need.

Have you looked elsewhere in the house for replacements? In my house the entire unfinished attic is done in the same flooring. I have pulled boards to use in the house and replaced the attic floor with plywood.