One part that you can pretty much never find is a replacement platen. The rubber on a lot of these old machines dries out and hardens over time, and can develop cracks such as the one in the photo. This typewriter was made by Remington in 1895, and is one of my current projects.
I won't delve into the details of all the work that has gone into this project -- instead I will focus on how I renewed this old cracked platen.
Step 1: Remove and measure the platen
The next step involved carefully measuring the diameter of the platen. I picked a spot where the rubber was still intact and used a caliper to measure the diameter.