Where and How can one find medival manuscripts?

The card catalog at Yale University's Sterling Memorial Library By No machine-readable author provided. Rageross assumed (based on copyright claims). - No machine-readable source provided. Own work assumed (based on copyright claims)., Public Domain, https://commons.wikimedia.org/w/index.php?curid=765557

Dr Sanchez-Prieto continues by explaining how manuscript collections first began.  Unsurprisingly, given the climate of literacy at the time and the tremendous expense of producing manuscripts, churches and church-bsed universities had manuscript holdings.  This continues to this day. 

Eventually, however, royalty also commissioned manuscripts, building private libraries.  Manuscripts were more or less portable, easy to give and collect, so they became popular artifacts and status symbols.

As the merchant class grew over the centuries, through industrialization, wildly wealthy philanthropists such as Pierpont Morgan were able to amass a good amount of authentic medieval manuscripts.  Private collections may or may not become available to researchers.  

The overview of manuscript studies would not be complete without mentioning the darker parts of history, specifically Nazi influence.  In service of Hitler's cultural purity and terror campaigns, many books were seized and destroyed, including but not limited to medieval manuscripts. 

On a more venal scale, there is the case of membra disjecta/disiecta: fragments of manuscripts.  In some cases, the membra disjecta came from manuscripts that simply fell apart.  In other cases, they were re-used in pre-modern times to bond other manuscripts or even provide insulation in clothing.  It was not unheard of for collectors to slice apart manuscripts and sell off individual membra, thus maximizing the potential profit of an individual manuscript.

Today, medieval manuscripts can usually be found in the libraries of large universities all over the world.  Digitization projects are numerous, but due to lack of funding, manpower, or other resources, they are often abandoned.  Internet portals help remedy some of this fragmentation (!!) by allowing scholars to search through the holdings of multiple libraries online.  But information can still be hard to come by as the means to catalog and even the standards of proper cataloging vary from country to country.  In recent years, some institutions have even turned to crowd-sourcing to help catalog membra disjecta.