I don't know if 5687 are still made, but there are tube manufacturers, Shuagang and Linlai in China, JJ Electronic in Slovak Republic, and Reflektor Corporation in Russia.[1][2] But 5687 are not rare.[3]

[1] https://www.thegearpage.net/board/index.php?threads/current-...

[2] https://linlaiglobal.com/

[3] https://www.ebay.com/sch/i.html?_from=R40&_nkw=5687