Found some on ebay but they are a little different. The ads say they fit my 108 but the pads don't look like your picture. They have a hole instead of a slot (mine look like your picture). Might not matter though so long as they bolt in place. I'm looking into using rear calipers off a nissan or mazda (or whatever) with an integral parking brake and using that parking brake with the cub arms to actuate it. Seems like a bracket for the caliper to mount up might be all thats needed...or I might get lucky.
Yeah, there are three different styles shown in the parts manual. If this tractor was just for me, I wouldn't even bother with it. I'm on flat land, and with the hydro, I really never use the brakes. Plus, the right side still works. But since it's the grandson's tractor, and they're on a hill, I better do it right. Good luck with your nissan/mazda experiment. Let us know how it works - so many parts are going NLA, it's always nice to find something else to do the job.