There are surprisingly few postcard collectors' sites that focus on a certain type of building (as opposed to topographical collections or sites on other topics).

Here's one about another type of cultural building: Joachim Hassel's collection of postcards of libraries at

https://bibliothekspostkarten.wordpress.com/

(organized as a wordpress blog, but easily navigated by categories)

Here's the only theatre within a library that I could find in my own collection – the Washington Folger Shakespeare Library:

http://www.andreas-praefcke.de/carthalia/id/1578