The phrase "an extra source for" is correct and usable in written English.
It can be used when referring to an additional resource or reference that provides more information or support for a particular topic or argument.
Example: "The article serves as an extra source for understanding the complexities of climate change."
Alternatives: "an additional resource for" or "a supplementary source for".
The Regents also put off the development of new English tests for 9th and 10th grades; the tests were intended to provide an extra source of information for the new evaluation systems for teachers and principals.
Foreign buyers provide an extra source of demand for swanky properties.
Extra point If nothing else, the Jets serve as an extra source of motivation for the Giants, and nobody inspires them like Rex Ryan.
The rise in the price of food is an extra source of stress for households who are already struggling to make ends meet, new research has revealed.
* 50 Cent, Eminem and P Diddy all have their own clothing ranges, while P Diddy has also carved out an extra source of income for himself in promoting the vodka brand Cîroc.
But, in a time when the economy is doing badly, these businesses seem to be thriving - doing really well - and provide an extra source of income for many people.
Therefore, we hypothesized that in S. pneumoniae phage-mediated lysis of a fraction of the bacterial population within the biofilm could provide an extra source of eDNA for incorporation in the biofilm matrix.
However, the paralogous loci found here are already an extra source of evidence for the genomic differentiation among our Berberis taxa.
From about 12 hours to 1 2 days after wounding and some hours after the onset of migration, there is a marked increase in mitotic activity in the basal cells a small distance away from the wound edges [9] [10], [12], providing an extra source of basal cells for the supplement of the advancing and migrating epithelial tongue [55].
