A Work allows External Links to a Score, but only on a whitelist of sites that are free to access. I think there should be an option to add an external link to a URL where you can "Get the Work" commercially, either as (a) a hard copy (b) downloadable score.
This is comparable to a Release having an external link to a URL to "Get the Music" by (a) purchase, (b) by Download. This inadvertently suggests that performers are able to be compensated for their releases, whereas composers can only give their work away for free.
As an example, consider the work Zebra Crossing: http://musicbrainz.org/work/faac3f2b-61a0-4f2a-a6d9-825fbd1c126b
If I want to purchase a hard copy of this score, I think there should be a link to the Publisher's page for the work: http://www.astute-music.com/shop/saxophone-sax-&-piano-zebra-crossing-p-82.html
Some publishers offer works as digital downloads only, and I would be inclusive to include these, eg. http://www.digitalsheetmusic.org/
I declare an interest with Astute Music publisher mentioned above, whose web site I host for financial gain.
Note, this is based on discussion in the forums: Feature request: Work external URL "Get the Work" (Score) at https://community.metabrainz.org/t/feature-request-work-external-url-get-the-work-score/153036