The Sun AX delta seat height is listed @ 20". The Terratrike Rover tadpole seat height is listed at 16.5" Standard chair seating surface height is usually in the neighborhood of 18".
Any tadpoles with handlebar steering?
You'll find several manufacturers in the links section here. Not cheap.
All the decent looking ones were in Europe. The American links didn't satisfy me. I've been also looking at trike conversion axle kits, but I can't find one that is compatible with a 21-speed bike.