If you would rather not to buy the required texts, there are free ways to access them. If WUSTL Libraries do not have a copy, you may be able to borrow one through MOBIUS. If MOBIUS does not have a copy, you may be able to borrow one through one of the local, public library systems (St. Louis Public Library - SLPL, St. Louis County Library - SLCL, or the Municipal Library Consortium of participating St. Louis County municipalities - MLC).
Encyclopedias and bibliographies are good places to begin the research process. Not only do they provide foundational information, they can point you to be most well-regarded sources on your topic.
The following reference works are recommended for initiating research into Venice during the medieval and renaissance period: