LibraryMusicSource.com offers downloadable parts for orchestral music from composers from Abt to Ysaÿe (there’s no composer whose name begins with Z, that they have at least). Their usual prices are good, but they currently have a great offer good until June 30: you can download 40 sets of horn parts (or any other instrument if you like) for $20 ($19.95 to be exact). You can choose from a ton of stuff – Beethoven, Bach, Brahms, Bruckner, Dvorak, Haydn, Mahler, Mendelssohn, Mozart, Ravel, Schoenberg, Schumann, Sibelius, Strauss, Stravinsky, Tchaikovsky, and many more. Example: Richard Strauss, Alpine Symphony. This is ONE selection of your 40, and you get the parts (in pdf form; you have to print them out yourself) for Horns 1-4, offstage horns 5-7, Tenor tubas 1-4, plus Bass Tuba 1 & 2 (sic). 67 pages altogether! And you have 39 more selections to go! Your second choice could be Mahler Sym. 2, with parts for horns 1-10 plus offstage horn. 80 pages! 38 more to go!
After receiving a very useful comment (see below), I need to amend the above to say that the same (identical) scores mentioned and many others are in fact available from the IMSLP (International Music Score Project) for the grand sum of $0 in the same form – downloadable pdf’s. Library Music Source may save you a step or two by having all the parts easily accessible, but for only a little further ado, you can have them all for free. I am delighted to have been corrected in this. Check out the IMSLP at your earliest convenience!