A place I use extensively is
The largest free blueprint and vector drawing collection on the Internet - 20000 vector templates for sale.
I think they search the web and gather plans together. Quality varies greatly, all the way from tiny badly pixelated and useless drawings, to huge detailed multiviews that give you more information than you could ever use. They also have a premium service that sells cleaned up vector based plans, but they're expensive.
In general, I just use 3-views as a place to start. They tell me how long the fuselage is, how long the wings are, and in general where everything goes. I also gather as many pictures of the plane off of the internet as I can to study the shape. I've even been known to get up close and personal at air museums and airports to visually inspect the real plane.
Oh, and another thing about 3-views I've discovered. Just because there are a top side and front view doesn't mean they'll be usable as is. Usually the first thing I do with a 3-view is go in and make sure all 3 angles are the same scale. Then straighten them out so they're perfectly straight horizontally and vertically.