In plain English
FAR 52.251-1, Government Supply Sources, is a contract clause prescribed at FAR 51.107, most recently dated Apr 2012. The complete official text is reproduced below, verbatim, from GSA's published FAR source files.
Its text contains no sentence requiring insertion into subcontracts; see the flowdown section below for what that does and does not mean.
Does it flow down to subcontracts?
No flowdown mandate found in the clause text
We scanned the full clause text and found no sentence directing the contractor to insert this clause into subcontracts. That is a statement about the text, not legal advice: a prime contractor may still flow terms down contractually, an agency supplement (DFARS, VAAR, …) may add requirements, and clauses listed below (if any) may order this clause into subcontracts from the outside.
Where it's prescribed
As prescribed in 51.107 , insert the following clause:
Prescribing reference: FAR 51.107.
The official text, verbatim
As prescribed in 51.107 , insert the following clause:
Government Supply Sources (Apr 2012)
The Contracting Officer may issue the Contractor an authorization to use Government supply sources in the performance of this contract. Title to all property acquired by the Contractor under such an authorization shall vest in the Government unless otherwise specified in the contract. The provisions of the clause at FAR 52.245-1, Government Property, apply to all property acquired under such authorization
(End of clause)