Planar Speaker Asylum: Koss Model 1's show up from time to time; less than Acoustats, also KLH9s, Quad57s, JansZens, RtR by cfb

All under 1K. There are others too but forget the Beveridge, Stax and SoundLab as they all go for considerably more than 1K unless you're very lucky and run across a deal in which case it's a no-brainer, snap them up. Sometimes a Dayton Wright ESL will go for under $1,000 but usually for more. Note, if you run across a Dayton Wright esl the insulating gas has more than likely leaked out by this time, this will cause the speaker to arc at a lower level than it was designed to operate. The gas used is Sulfur Hexafluoride SF6, a common industrial electrical insulating gas for high voltage spark arresting environments. One can buy a lecture-bottle of SF6 gas but it's not cheap at $300 to $500. Despite the name, SF6 gas is generally considered harmless see; http://en.wikipedia.org/wiki/Sulfur_hexafluoride

Dayton Wright history,
http://www.dayton-wright.com/DaytonWright.html

Koss Model 1a, last pair I saw went for between $400 to $500, skip the Model 2's and 3's they are poor performers compared to the Model One or the Acoustats. Also the Koss will require a high current stable amp as they are finicky about amplification much like the Soundlabs.
http://www.audioflipper.com/model.php?id=109717

Hope this helps.