There are a number of very old and hence copyright free bestiaries but many of these have been scanned and uploaded and named as copyright even tho the work they are copying is public domain so I think personally (tho you would have to be sure) that these bestiaries could be used. Certainly you could add the obtained form notice that these places seem to want you to put on stuff. Also I recommend Liams old books which have some pictures in them which are copyright free. Also Ravs knows a shop in London that sells books of public domain images and I guess you could see if they have ISBN numbers and get a copy. One day I will have to visit that place with a wheel barrow in tow...

http://www.fromoldbooks.org/

and also wiki commons
http://commons.wikimedia.org/wiki/Bestiary