Nobody mentioned ZDNet (Edit: whoops; I did!), and CNet and Download.com are the same place. (Technically all 3 are CBS companies, but it seems ZDNet requires registration for downloads - I do NOT know whether or not they do the spyware wrapper thing that CNet is doing, but I suspect not.)
Anyway, the regular UltraVNC downloads page is at http://www.uvnc.com/downloads.html
There are no "official" mirrors that I'm aware of, but Softpedia seems pretty trustworthy (they actually appear to put in the effort to vet and CURATE their listings!), and I'm sure there are others (Snapfiles is probably okay, and maybe MajorGeeks, though they've pushed adware in the past). Some of the code is at SourceForge too.