I don’t know where else you could look but the common E-Book & Audiobook places. So far many Books were not listed in the usual piracy sites. I had to search explicitly for the only Book/Comic/Audio Book related ones.
These sites however have TONS of Material in all kinds of categories. So even if there wouldnt be a “Artbook” Filter, if you look for a reocurring comic, make use of the reocurring name and hit the search with it.
You may have to check what are the best sites for your language though.