Scratchmade Cars (smcars.net) is also pretty good. You'll have to register (free) to see the archive. It's not always the best quality, but the archive of blueprints and reference images is quite diverse. If you have an idea of the make/model and year, odds are good they'll have it.

I recall seeing a few other sites that seem to have had better quality stuff print-wise, but they were fairly biased to the high-end or European makes. (If looking for references for an American make and model that wasn't a classic or sports car, I remember being SOL at those places.) If I only I could recall the name of one of those (otto-something? It's been too long.), I'd recommend it if you were into Euro-cars. Simply because of quality/consistency of the prints.

And this sounds funny, but sometimes to tell if a website has good prints is to compare them to photos and overlap them. If they have orthographic prints, the lines will be different in some areas. (More noticable the further out you get from the center of the image.) If its a trace-over of yet another photo, it's more likely to have the same types of perspective distortions and be closer to your photo. (Perspective distortion tends to be bad if you're trying to do accurate work.) In some cases though, photos or photo-based drawings will be all that you'll have to work with. In which case it's going to involve more tweaking work for consistency across multiple viewing angles to get things close.

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.