Does anyone know of a decent online store where you can buy a wireless N adapter that works with the 360.. a bridge or gaming adapter etc? (im looking for an ethernet connection, obviously not USB).

Any router will do but be sure to get the same brand as the device you're connecting to (eg. the other router). Wireless N is still a draft and not every hardware understands all the others.
Just a Question... Why'd you need N? G is enough for HD movie streaming and gaming and a N router accepts connections from b,g and n clients except you configured it not too.

i was getting a real poor performance with my current G setup. However, I've since solved it, by changing the position of my router - so it's all good

Don't forget, wifi G with some kind of speedbooster will max out the xbox's NIC.
wifiG-108mbps NIC-100mbps