Hi,
I don't know any "dedicated websites", however, the likes of What Car, Autoexpress, Top Gear websites and associated magazines give details of when various models are facelifted or changed radically. Plus, the likes of Parkers car guide magazine also gives relevant details etc etc.
For example, the latest Honda Civic is unrecognizeable to its predecessor, the current Nissan Micra and the new Ford Ka, to name but a few, are also visually different to what they once were.
Present car shapes usually last at least 4/5 years before anything radical happens to its shape/appearance. Therefore, beware a model which you've seen around for that length of time - it's due for a change!
Good luck.