Keep an eye on Ebay for a cheap set of these Saas mach 1 seats:
Shouldnt matter how tatty they are. The have an integrated head rest which can easily be lopped off (it is a separate loop on the frame) - and get them recovered in leather. Should come in less than your budget. I know of quite a few people who have done this - it is quite a narrow seat and I reckon would fit in all cobras.
Cheers