For starters, I would avoid music written for any of the Bb instruments. This would include Trumpet, trombone and tuba, and maybe a few more.
Music for any C instrument will work, which is a lot of leeway. Guitar, mandolin, banjo, fiddle/violin, piano/keyboard, and I think all the woodwinds are C instruments. In some cases you may need to shift up or down an octave to reach the lowest or highest notes in the sheet. For instance, a guitar in standard tuning can play notes almost an octave below the dulcimer tuned to DAD. To play these sheets, you may have to shift everything up an octave on the dulcimer. Not really a problem. The fiddle may have more high notes, and less low notes than the dulcimer. You may find some songs require you to play an octave below the fiddle. Mandolin has the same tuning as fiddle, and the same range. This is really not an issue, you can adjust the octave. Music for the 5 string banjo may be closest to the normal range of a DAD or DAA dulcimer, but the 2nd, 3rd & 4th strings of a guitar are right there as well.
Finding dulcimer books is easy, if you are close to a festival, or a store that sells dulcimers. If not, try online. There are several places doing online/phone sales. Prussia Valley Dulcimers ,in Waverly, Ohio; Elderly Music in Lansing Michigan; and Amazon.com all carry dulcimer books. Also, the major music publishers have web sites you can check for music. Hal Leonard and Mel Bay are probably the two biggest in the US, I'm not sure who covers other countries. Looking for specific types of music can be tricky, though. Many books are written as instruction books, teaching how to play. They usually contain older folk songs, as most of these are in the public domain, meaning the author/publisher can use them without paying royalties. With modern songs,royalties must be paid. There are books focusing on some modern songs, but you pretty much have to see the table of contents to decide which books you want to buy. There are books on swing, blues, classical, rock, and probably more. A festival is a good place to look for books, as vendors will bring a lot of books on a lot of musical styles. And will probably know what is available that they haven't got on hand.
Also, maybe FIRST, try the tablature section at Everything Dulcimer. You may be able to download or print some of the songs you are looking for. http://everythingdulcimer.com/index.php Again, copyright and royalty issues may make modern songs harder to find.Tablature is in some ways better for a dulcimer player, as we sometimes require different tunings. You can learn where all the notes are, but once we change the tuning, the notes move. And some notes are lost and gained with each tuning change.
Paul