Transmission Line speakers

Lynn Olson's Ariel, if you can find the drivers. Alternatively, the Seas Thor kit if you want something with more grunt. However, I really wouldn't build the enclosure as-is, but one of the alternative cabinets for it, which work better (as found by several Thor owners who have tried the new boxes & found a considerable performance gain).
For the Ariel (first hit when you put it into google). You'll have to price up the drivers & XO components yourself though -it's not sold as a kit)

For the Thor,
(first commercial seller when you put it into google)

The different cabinet designs are found on the Clarity on Seas Thor thread in the Loudspeaker section on this forum. Quite an interesting thread actually & worth a read.