Spink sold the Nissen reconstructions of plates 1-11. Before the sale, they scanned them and made them available on their site.
If you can't get there with a link to the lower page, you can reach the scans by:
going to spink.com
selecting "Private Treaty" from the menu bar
selecting "Nissen Reconstructions" from the menu bar
There are a few errors in the plating, but overall, just about all of them are correct.
For later plates, you'll be best off getting the CD of the scans of the imprimatur sheets. For more information and a list of references, see:
http://www.sturstamps.com/help.htmoh yeah - if you happen to have an imperforate with corner letters "CG," you can check this site:
http://victoria.cgpostal.com