I am having a time finding a saddle for my horse. I came across this tool...

https://www.horsesaddleshop.com/mihs-saddle-fit-tool.html

I did.   And the only saddle that they said would fit was a draft saddle.  My horse was wide, but not that wide.  Waste of time and money, IMO.   

It was hard to use as well, as it was very loose and so getting an accurate reading was difficult.

I haven't used it, but I have one I'd be willing to part with for cheap. I've played around with it, it is really cool. 

I can't speak for the tool, but I once saw someone baically made a paper mache mold of their horse's back and then took that to the saddle shop to "try on" saddles, since they were unable to haul  the horse (I believe it was just too far of a distance but I don't recall for sure). It was a really neat idea. They just flipped the saddles upside down and then placed the mold in it and saw exactly how it fit.

One problem with the tool mentioned is that it only seems to measure gullet/bars from the FRONT of the saddle, but doesn't address the rest of the saddle.