If a particular library doesn't have one, you can write it yourself. In that case theres no need to model the entire library - you can only model the subset of functions / methods that you use.
Or what if, hear me out, everyone just did that thing from the start?
I do tend to start all my new node projects in TypeScript, yes.
With TS do I have to have a spec for every third party code or can I import something and use it as-is, but not get the benefits of TS on that part of the code?