I don't know of any "open-source" tab sharing sites (i.e., sites specifically dedicated to CC or other permissively-/share-alike-licensed transcriptions) but for what it's worth there are a large number of "free" (as in beer) guitar tab repositories, or at least a large number of places that include freely-available, user-contributed content within a broader freemium or ad-supported service.

Beyond the usual web-based suspects (some of which have been named elsewhere in this thread) there are also a number of other tab sharing "communities", including some relevant subreddits that might be the easiest way to spread the word if you're not comfortable uploading to a more directly for-profit service (not that Reddit isn't for-profit too, but guitar tabs certainly aren't their business model and you'd largely be linking to files stored elsewhere rather than uploading them directly).

But a lot of people seem to do what you did also. I suspect it's not super-effective as a distribution model, but if you dig a little it's not hard to find smaller/specialized tab collections on personal websites or general-purpose file hosting services like Google Drive/Dropbox or even at places like Github.

I don't think Google's `filetype:` operator respects guitar-tab-related extensions like `gpx` or `gp5` or `tab` or whatever but using the `inurl:` operator (or just including the tab-specific file extension) combined with the song title or artist name is a good way to find some of these less-centrally located tabs.

Also there are several massive collections available via torrent and on some of those more gray-hat-ish anonymous/one-off file-sharing services. Arguably these may have a more questionable provenance but I suspect the plurality if not the majority of the guitar tabs you'll find shared anywhere online could be traced back to old-school usenet or the first generation of tab-sharing websites that grew out those communities. Ultimate-Guitar for example self-reports as having "chords and tabs to over 1,100,000 songs", and likes to highlight their "official" (licensed) transcriptions (and to be fair they are nothing to sneeze at), but it's probably no coincidence that they don't seem to report the ratio of "official" to "community-provided" transcriptions.

EDIT: Just out of curiosity I checked the file-count reported at the bottom of UG's search page when filtered by file type, I see:

* 18,688 "official" tabs (AFAIK these are the only actually licensed transcriptions)

* 212,408 "Guitar Pro" (.gpx/.gp5/.gp4/.gp3) files; user-generated and many of which probably weren't originally uploaded to UG

* 1,115,129 plain-text transcriptions (801,597 "chords", 313,532 "tabs") that almost certainly date back to Usenet-era communities

The ~20K of licensed/official transcriptions are impressive (both individually and collectively) but represents less than 2% of the overall song catalog (and much less than 2% by file count).

I don't know if it's the case any more, but I found that in the past many of the "official" tabs were actually just the highest rated Guitar Pro tab converted to use the web player and dressed up to look fancy. With a little bit of effort you could get the same result just by looking for the highest-rated gp5 and using that locally.