I used to know the sites pretty well--Langley has images, but I can't find them quickly right now.Originally Posted by Tom Hoskinson
Most of the more famous ones can be found at grin (Great Images in NASA). They have some very high res scans.
Johnson space center (jsc) has an archive as well
I think this is the one I remember using the most. The have the 3MB scans of each image.
with this one for Apollo
This site lists the different archives: