Judy, most bluegrass tunes are the same tunes played in dulcimer circles or old timey groups, but they are simply played much faster in bluegrass groups. The most comprehensive collections of those tunes in dulcimer tab are Stephen Seifert's Join the Jam books. You might pick up the first of those books and then do an online search for common bluegrass tunes to see which ones are played in both circles.
One thing to look out for, though, is that while dulcimer players play nearly every tune in D, in bluegrass circles, other keys such as C, G, and A are also common. Blackberry Blossom is always in G, Billy in the Lowground is always in C, Red-Haired Boy is always in A, St. Anne's Reel is always in D, and so forth. If you are going to play with bluegrass folks you will want to learn the keys for common songs.
When I was learning mandolin, I used a book/CD set called Steve Kaufman's Bluegrass Workout which has music for a whole bunch of common bluegrass tunes, with a CD that plays each tune once very slowly and then once at bluegrass speed. But each rendition has the full song once and then two versions with just the back-up instruments, and you are supposed to play the melody. It was really good practice. The book does not include dulcimer tab, though, but if you can learn by ear or get the basic melody off the standard music notation it could work pretty well.
