First, of all, you might be able to repair the old one. Sometimes if the switch won't stay in a position, it might just need some "gentle persuasion" by slight bending of contacts. If this doesn't work, it will need to be replaced.
Ideally, if you can find an original Switchcraft right angle switch, it will look like this one below. Note that the knob is a metal slightly flat concave top knob. Also, the original knurled edge thumb nut that holds it to the surface is larger than the newer variants. Unfortunately, these parts are uncommon and tend to be sold on eBay and reverb as part of entire wiring harnesses, which tend to be pricey.
It needs to be a right angle switch to accommodate for the shallow depth of the body, but you might also be able to fit a short frame, thin body switch in your T-100D. Be sure to check measurements first.
You can find new non-original replacements from
Switchcraft.
If you want to hunt down an original part, try
Hans Moust or
Ken Nash.