Last updated 01/15/2018
Ralph, W0RPK, maintains a WORKED-ALL-STATES list of US stations that are willing to complete two-way QSO’s AND QSL for future WAS awards. That list can be accessed here.
Note: The link is not static and is updated regularly so beware if linking. It’s best to link to the parent post if embedding.
This list, originally maintained by VE7VV and detailing North American stations that have completed at least one two-way QSO on MF and/or LF and are willing to send QSL’s, is no longer maintained after January 1, 2018 and may become inactive at any time.