Replacement Parts
Metal Parts
Most metal parts that have issues won't be broken, just bent. Instructions on forming/bending parts back to the correct shape while minimizing risk of marring or breaking parts can be found [not here yet]. Metal parts that have failed most often need to be sourced from parts machines, or machined by a machinist or machine shop. Otherwise, 3D printed plastic parts can provide a temporary solution until the part wears out, depending on use of the part.
Springs
Springs are considered "metal parts," but are normally very generic and can be replaced more or less approximately. Many "weak spring" problems aren't actually any fault of the spring, but a slow/sticking pivot. In the case that the spring has actually lost tension, loops of the spring may be cut off and new loops folded to restore the original shape of the spring, but shorter and with more tension as a result.
Rubber & Plastic Parts
Almost all rubber parts are susceptible to deterioration simply due to time. This includes platens, feed rollers, grommets, bumper pads, and feet.
Grommets, o-rings, and bulk rubber sheets can be very useful in replacing old rubber. For more specialized rubber parts, 3D printing in TPU can provide suitable replacements.
A list of 3D files for parts organized by Theodore Munk can be found at https://3dtypewriterparts.com/.
More parts can be searched for by using the 3D model search engine at https://www.yeggi.com/q/typewriter/.
Parts made by user @dairyking98 can be found at his shared Google Drive folder for typewriter models here.
Sources for Parts
Potential suitable replacements for hardware, springs, grommets, spacers, bumpers, and more may be found available from https://www.mcmaster.com/.
Drawbands
For flat drawbands, twill tape and edge tape seem to be good keywords for a material suitable for replacement.
For cord-like drawbands, kite string, heavy duty fishing line, and hemp wick have been known to be suitable replacements within the typewriter community. It is advisable to maintain a similar thickness in the replacement as the original.
