It is a bit trickier to do a complete search now that the main site and the gallery site are separated. Still, the best place to start is with the Browse By Category section at the top of the middle column on the main page. This will narrow down the choices. The search box will search the entire oldbookart.com site, but will not include the gallery.oldbookart site. For that you will need to do a new search on the gallery site.
Sorry that it is more complicated than it should be. This is one of the main drawbacks of the new site design.
You can also try a Google Search which should search both the Main site and the Gallery site. Google does a pretty good job of staying current with the content here, so using the search box below may bring the most comprehensive results, although it may miss some newer content.