It seems like all I ever do here lately, is link to those two sites with registration number, but what the heck...
I think you'll be best off taking a look at http://www.intercpt.demon.co.uk/fleets.html
. It's not very up to date, mind you, but it's easy because the fleet are organized by country and airline.
Once you know which types of aircraft are in a specific airline's fleet, you can check for more recent info on http://www.bird.ch/bharms
Some people may be getting bored of seeing these links all the time, but they're great, and hey... if it helps you out! So enjoy your search.