Well, I hope every TS developer is joining the effort to provide definition files for everything at central repositories like definitelytyped. So I always start looking there for an existing one and update it if necessary. If none exists I write it myself... and send it to definitelytyped when it's complete and tested successfully.
Supplementary files are a bad idea. Are people really that afraid of pull requests? Withholding fixes does not only thwart the idea of open source, but it's also a technical debt in a project. Major upgrades of 3rd party libraries can already be a pain, but they're even worse if you need to upgrade your private fixes for the 3rd party libraries, too.
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