Janoskiss always recommends the following. He's what he said earlier:-
"Best barlow for high powers IME is the fully baffled Orion Deluxe 1.25". Got mine for $99 from Sirius Optics in QLD. An absolute bargain."
Not sure of current price. OK, just looked it up:-
http://www.sirius-optics.com.au/barlowe_pieces.htm
$120.