You could argue the 30+ corpses by the Arx waypoint are a source fountain (depending on your point of view of course).
In any case source in source fountains has to come from somewhere presumably before it is stuck in the bottle. Don't ask, don't tell perhaps.
I just suck it up from anything that happens to be around...
And yes there is a proper source fountain in Act 4 - it is next to where you find Arhu the ex-cat in Kemm's basement but it is a bit late on to be useful.
