Softpedia seems to have 1.3.77 both 64 and 32 bit versions: [link removed]
If you're trying to install via PPA I'm not sure.
Edited by GreatMarko, 06 April 2014 - 07:47 AM.
For your safety, Sync should only be downloaded through official BitTorrent Sync channels. For 1.3.77, please use http://syncapp.bittorrent.com/1.3.77/