Is a n1 speedlight compatible with a nikon dslr?

I need a new speedlight for my d90 and i'm on a tight budgent and the n1 speedlights seem cheaper. Are the compatible with my d90?

No
If you want to go inexpensive, you need to look at the Nikon SB 400

Do you mean the speedlight for Nikon 1 SB-N5? The SB400 is $30.00 cheaper then SB-N5.

Forget the SB400 - woefully underpowered & no bounce/swivel head.
Have you considered a manual flash? Usually a lot more reliable that TTL & it's not hard to learn how to light manually.
A Yongnuo YN560 costs around £45 and a set of radio triggers to fire it off camera (the best place for it) costs around £20.