One source is goody huts.
Another source with BTS if random events is on is some random events (but there are others that cost money)
The primary sources though are:
1. Religious Shrines; especally when combined with all money multiplers.
2. Planting cottages which grow up to evenually become Towns; (this replaces roads in previous Civ game)
3. Merchants & Priests add gold as well.
And Courthouses reduce city & civic upkeep.
In addition, playing an Org civ would reduce your upkeep and provide cheaper Courthouses and a few other buildings while playing a Financial Civ will give you an extra gold in every tile already producing 2+.