If you're after original parts then try: http://www.eurospares.co.uk/
If you're after replica body panels or parts and the occasional few used bits-n-bobs then talk to DC Supercars: http://www.dcsupercars.com/
For other small bits-n-pieces, if you look over on the LamboReplica.co.uk forums, in the Countach section there is a thread that has been going for a few years where people have been identifying "common" parts used, i.e. Fiat and Alfa parts.
That will allow you to find loads on e-bay
As far as the glass is concerned, i know that Pilkington Glass UK has a specialist section that has 4 or 5 templates for different Countach kit Glasses from over the years. Again i think the contact details for them can be found in the Conutach section on the LamboReplica.co.uk forum
I hope that helps?