The XR owners club forum (as RS posted) would be the first place to start. There will be some lovely examples. They will be top money though as enthusiasts tend to be blinkered and think the values of their cars are more than Joe public. There will also be polished turds! Being in the scene myself you see some cars from popular forum users that everyone rants about that are quite honestly not all that. It's no different to a BB forum in that respect and I'm sure you get that point 100%!
Ebay is always a good window front. Oldskoolfords.co.uk. passionford.com. rsownersclub.co.uk. Turbosport.co.uk. Fiesta club of Gt Britain (don't know the site). Numerous places to hunt them down. Just take someone with you that knows what they are looking for. There will be a registrar on the XR site that will have a database of alot of the more known motors.
RetroRides has been mentioned but that tends to be more of the poverty models for cheap retro hacks. There may be one or two on there though.
If after an Escort the early XR3's are the desirable ones. Pre injection. Hard to find now though.
Sadly the XR's don't have the following that the RS's have. Even that is dying out now as people have less flash cash.
I know I have whored some of my cars before but as RS is after some pics, these are a couple of my rencent ones..
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My shed is currently full.. Working on them is part of the fun, you ever need anything doing...