I just got an early 80s Peugeot PKN10. It is a 57cm in decent shape but the only thing that makes it not rideable for me is the stem. It is way too short and I feel bunched up on the bike. It needs at least a 100mm stem. Where can i find a stem that will fit this old Peugeot?

First of all you need to get some precise measurements of the stem and bar size with a metric caliper. If you do not have one, then go to an independent bike shop for help. My guess if it is an early 80's Peugeot is that the stem size going into the fork is a standard British size, which would make replacing the stem pretty easy. I am not sure as to the bar diameter, this could be British or French. Have both measured and go from there. I think if you go to a long established bike shop they should have a 100 or 120 mm extension stem available. If not, there is always some junker bikes to get parts from. Good luck.