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

See slate.com

"The Stunning Grandeur of the World’s Great Opera Houses"

"Tarjetas postales" ("Postcards") is the title of five small piano works by Joaquín Turina.

1. Danza Vasca

http://www.youtube.com/v/4M2W6rPjukU

2. Ramblas de Barcelona

http://www.youtube.com/v/TmheSR-ka2k

3. Madrid

http://www.youtube.com/v/HxRWhAJNZCc

4. Paisaje Granadino

http://www.youtube.com/v/3aq5QsGnYds

5. Romeria

http://www.youtube.com/v/HTQTdgs5eEQ

Pianist: Antonio Soria