The only place I have seen them is eBay and on Zilvia classifieds. Also post in Facebook Groups like 240SX Parts Exchange etc.
Quarter window stuff is among the first parts to sell when a vert is parted out so you have to be quick.
Also there is a guy who sells a repair kit on eBay for the guides. He can install them or you can do the work. Search for "240SX convertible window".
http://www.ebay.com/itm/1992-1994-Nissa ... a0&vxp=mtr