If you get no luck on here mate try BSC

http://www.buysellcortina.co.uk/forums/

TBH most come up on ebay but any that do are usually posted up on there as well
Any idea what your after? estate or saloon, pinto or V6, you gonna mod it? if you fancy going for a V6 start with a V6 it's so much easyer.
If your gonna mod it lots and aint an origionality freak start with a GL, cheeper than a Ghia or idealy a 1.6 L, the cheapest and already a pinto so a 2.0 will drop straight (ish) in and good ints are cheap, £40- £60 will get you good cruisader int thats the same as a Ghia but you'll pay £500+ less for the L to start with

Mk4's are probably the rarest Mk left at the moment but still not more than a Mk5 just harder to find

Good luck with the search