I also had the same problem with that site. The Montana part numbers all seem to start with an A and when you search the number there is nothing found, and I couldn't find any kind of LS to Montana part number cross reference. This is what I came up with to cross the numbers over, but you need to know what model LS tractor your Montana is.
I started by looking up the part for the comparable LS tractor on this site (ls-tractor.com/parts.html) in my case the Montana is an R4944 and the LS model is an R4020. That link has part look ups for LS and Montana tractors. After I found the LS part numbers (click the series type on the left side icons) I went back to Montana page for parts number look up and typed in the LS part number (doing this was originally a mistake) and the look up showed the Montana part number, the LS pars number and the price. Every part I looked up matched. This seems cumbersom but it's what I found to work.