OK I think I've got something here.
Yes, the link does seem to be missing from the US Store.
I tried poking around in My Account, and Purchased pages, but no luck.
What DID seem to work was Ping.
While not as nice as the old Complete My Album page, it still works.
You can see in your Library your single tracks.
Highlight a previous purchase, and click the oval Ping button to get the dropdown.
Under "Show in iTunes Store, " choose the name of the song, and it will open the album page.
From there you can see the song as "Purchased" and the others will have a "Download" button.
The price to complete the album seems right. In this case, $8.70. I logged out and searched the same album, and it's $9.99.
Hope this gets us on the right track, and I call "shennanigans" on Apple for not making this solution easier to find. Such as putting an update on the old Support pages for Complete My Album.