Should be in a package hanging on one of those trees in the electrical section. 8 bucks?
Easy enough to hotwire as well if you need to get it running, for ease and if you know you are getting another keyed switch: just drill out the keyhole nicley and use a screwdriver to turn it or disconnect the switch and jump the grounds and the hots but that gets tough and dangerous if you have to use the machine.
If there is a bigger rental yard or eqipument shop around ask if they have a key but that don't always work. I got one for my Dingo there, its a different kind of switch, more basic but I had to lend one of my chipper keys to this numbnut who lost both of his and it fit. I had to keep calling and finally demaned he give me the key back cause lost keys are a pain in the ass as it seems you allready know.
I managed to lose the spare keys to the Dingo again which also had the lock keys so if I ever lock it in someone's yard and lose the keys I have to haul the torches or something out to cut the locks off and ask the rental store for another key.