Do you have a Montana parts dealer nearby? I'm sure they could get a key.
If not you may try to contact an LS tractor dealer. LS built your Montana. I understand that since LS started marketing their own brand of tractor that they no longer make Montanas.
Post a pic of the key. Maybe someone will recognize it being the same as theirs.
See if yours looks like one on the list below. HOME OF THE EQUIPMENT KEYS - WWW.BEACHSLOCK.COM
The key for your tractor isn't really a security type key, it is meant more as a safety feature to keep kids or other unauthorized users from starting the machine easily. Generally all the keys are alike for a given make/model. You may find another switch that fits the hole in the dash will be the quicker/easier fix.
Thanks for the help guys. I just found some on eBay. They were being marketed as a new holland key, but I'm almost positive it's the right key. I think LS makes some of the new Hollands, so I guess it makes sense that they could use the same key.
You are right, LS has made many New Holland tractors. If it looks right, it likely is. Congrats on a good find, I hope it works. Please post back with the key you used and the end results so future readers know the cross reference exists.