One way to boost sources without increasing the total income in a province is to add a third income type - say "mebhaighl points" or "MP", these could be used to sustain a source-based court, substitute for GB in realm spells and source actions, substitute for RP in spells, etc but not converted to GB or otherwise used to interact with non-source holdings.
That would leave the source holders with "income" so that they weren't totally dependent on other regents (good and bad points), but doesn't work too well for mixed domains (although I suppose you could say that MP stacked with GB for court purposes on any court actions relating to realm spells and source holdings/ley-lines).
Otherwise I'd suggest reducing the RP cost of spells - though that would likely benefit temple domains (although as I seem to spend my RP on spells which help other people I'm not sure if that's a bad thing!)