The phrase "are an essential source" is correct and usable in written English.
It can be used when discussing something that provides necessary information or support in a particular context.
Example: "Books and articles are an essential source of knowledge for students pursuing higher education."
Alternatives: "are a vital resource" or "are a crucial reference".
Except that property taxes are an essential source of money for public schools.
They are an essential source of timely and reliable information for the public.
Second, postdoctoral appointments, which are generally filled by researchers who have recently earned Ph.D.'s, are an essential source of candidates for permanent positions.
Mr Hastings believes that the humble DVD and, eventually, high-definition versions of it will remain popular for some time, not least because that is what the movie industry wants: sales of DVDs and fees from rentals are an essential source of the studios' profits from new releases.
As a consequence, patents are an essential source of information for research scientists.
Moreover, inhomogeneities in the RF field are an essential source of error for quantitative MR spectroscopy.
If this NATO is "an essential source of stability in an unpredictable world," heaven help us all.
Yet Ms Gessen's book will be an essential source for anyone interested in the rise of Mr Putin.Marie Mendras's "Russian Politics" is more academic.
"City AM has been a daily read for me since I came to London seven years ago and it's been an essential source of news and comment throughout my time at the Institute of Directors," May said.
The World Sanitation Financing Facility has been created to devise financial structures to enlist private investment in seven areas from public toilets to fertiliser production.The financial industry may be providing the rocket science, but the backing of rich philanthropists is an essential source of fuel.
Groundwater is an essential source for drinking water suppliers.
