during the act1 (when you first get them) you also have a source fountain in the Armory. so you have infinite source between combats and you can use them in every battle.
during the act2 you also have an easy access to a fountain in the meistr's cellar. so again you can use them in every battle.
during the act3 it's a bit more complicated. there is no fountain, but there are plenty of corpses and spirits. also there are very few battles you really need to fight. so I guess again you can use source spells in every battle.
act 4... not really sure if there is a fountain(even in DE it was the fastest act to finish), but I usually tend up eating corpses\spirits\steal source during combat anyway...and with apotheosis I had no problem casting source spells every combat.
depending on a character I usually either go All-In (3sp on turn 1) or have some party members use 1sp spells and save for skin graft.
and yeah, I think source concept is dumb(especially since you have infinite time between combats). imho it would've been better if there were no source. just "once per combat\rest rule" much like daily spells in trpgs like D&D
