the extension housing seal is the transmission rear oil seal; i.e. *not* the seal you are looking for. for a ka24de (straight from an old order status):
Mechanical Catalog - Nissan
Engine, Crankshaft and bearings, Rear main seal, REAR MAIN SEAL, 240SX - KA24DE 1 $13.13 $0.00 $13.13
Status: Sent
i had the part number on the bag it came in but i threw it out.
you can order it from everythingnissan.com, or any of the other major nissan parts website if you just do a search for "rear main seal." everythingnissan.com has a diagram of it on their webpage when you look up the part. consider printing it out and taking it to your local nissan dealer if you don't want to wait for shipping.
for something like this, i would use OEM. going with a crappy 3rd party seal is fine on something easy to replace like the tranny oil seal; but this one is kind of hard to get to. it's not like it's that expensive anyway.