Sources for Photoplay Music
This music has been out of print since 1928, so it can be hard to find. Keep your eyes open, watch eBay, visit nearby university music libraries, and ask about their special collections. Be very nice to the librarians! A few places have finding aids on line, but -- and it's a big but -- you need to know what you're looking for, and the titles are rarely an indication of the quality or usefulness of the piece. Browsing through the actual music is the most efficient way of searching.
The Silent Film Sound and Music Archive
A growing collection of scanned cue sheets, sheet music, and audio for silent films can be found at the Silent Film Sound and Music Archive. Searching is awkward, but many of the pieces are available as downloadable PDF files, and quite a few are arranged for full orchestra.
University of Colorado, Boulder
The American Music Research Consortium A finding aid for the Alvin Layton collection is online as a PDF file.
University of North Texas Collection
You can search the collections of the University of North Texas by author or by title. I've had good luck ordering music from UNT for the costs of photocopying and postage.
University of California Los Angeles (UCLA) Music Library
The Music Library has several collections, not all of which are on line.
The Capitol Theater (New York, NY) Music Collection claims, unfortunately, to consist mostly of piano parts. With its 71+ piece orchestra, one would have expected the full orchestrations; and being one of the premiere venues in America for live-orchestra film (directed by Erno Rapée, William Axt, and David Mendoza; all of whom were involved in publishing the Capitol Music Series), it should be an excellent collection. Note that some of the Capitol material is at the NYPL (see below), and this finding aid does not give instrumentations or copyright dates.
The Schnauber collection (originally from the Film Music Society) is stored off-site, and when you place an order they have to send someone out to find things -- so it's smart to place one large order rather than a bunch of small ones. The catalog is particularly detailed, and has authors, titles, and arrangers. If you click on a number, it will show you the orchestration, and often the catalog includes film cues that are pencilled on the first violin part.
The New York Public Library
The New York Public Library has a collection that originally belonged to the Capitol Theater in New York. (I don't know the relationship between this and the Capitol collection at UCLA.) I haven't tried to get things from here yet. If you do, please report to me how it went.
Southern Illinois University of Edwardsville.
These are very helpful people. The Rader collection leans heavily towards classical music, but photoplay and popular music are scattered throughout. There are several other collections at SIUE that are not on line yet.
University of Pittsburgh
The collection at the U of P includes a lot of orchestrated photoplay music in the Mirskey Collection as well as other music. As far as I can tell, you have to search their entire library for composers or titles you're interested in. Searching for "silent film music" (in quotes) brings up a lot of the titles. Use interlibrary loan to get scans of the pieces you're interested in.
The Dorman Hundling Archive at Drake University
Drake University's Dorman Hundling Archive is a collection from a Newton, Iowa movie theater, quite comprehensive. They currently charge $1 per page, and can scan to PDF.
Cleveland Public Library
There are some small orchestra pieces at the Cleveland Public Library, which also has a large collection of popular sheet music.