The "twofers" were released by Warner Bros. (which owns elektra) in the UK, and are sporadically available as imports in the States. Most of the ones you mentioned are out there, somewhere.
Collectors Choice (http://www.ccmusic.com) has been licensing and reissuing old Elektra folk albums in the States, but -- unlike the British releases -- aren't particularly bargains. Butu they may be easier to find. (CCM usually released their own compilations through the catalog and website exclusively for several months, then make them available through regularly retail outlets, where they may be less expensive -- if you can find them).