Yeah, I went through a remodel and dealt with the same thing, trying to get drywall to line up on old (1970) and new walls. There's more to it than that, but it's the quick version.

On the other hand, if you find a smaller mill, usually the ones specializing in hardwood, you can still get "full size" cuts, usually without even asking.

My house was built in 1962, and there were some exposed studs, and spares made into things like garage shelves. The old 2x4's are bigger than new 2x4's.