I have a book called "Bluegrass Gospel" with 30 songs and music for piano/guitar/four-part vocal which has a lot of the old standards. It's published by Hal Leonard Publishing, 1983.
Most of the gospel songs in my own songbook are ones I've collected from CD's, jams, other songbooks (the Blue Book: Rise Up Singing) even hymnals. You'll find a lot of gospel songs within bluegrass songbooks, because of course it's an essential part of bluegrass. Bluegrass festivals - the larger ones - always have vendors with songbooks for sale, and you can probably find unusual ones there.
Good luck. It's worth looking!