We really enjoy listening to the old spiritual-type songs, such as Down in the valley to pray, I’ll fly away, In the sweet by-and-by, and so on. Does anyone have any ideas as to where we can find more? We have very little, so any titles will be great.
Emmylou Harris put out quite a nice album of old-timey gospel called “Angel Band”, I think.
There’s out there. Go to a music store or site that specializes in folk music (for example, the Elderly Instruments website) and you should be directed to lots of good stuff from Jim and Jesse, Bill Monroe and tons of other folks.
The O Brother, Where Art Thou? soundtrack has several of them. Assuming it has all the stuff from the movie, I’ll Fly Away and Angel Band will be there.
I agree, the soundtrack from “O Brother, Where Art Thou?” is terrific. Give it a listen.
For me - anything by Tennessee Ernie Ford - he did some wonderful gospel/old-timey hymn albums (what I call my grandmother’s “kitchen hymns” - because that’s where I’d hear her singing them as she cooked) and quite a few have been re-issued on CD.
Searching Amazon.com for titles like “gospel” and “bluegrass” together will yield you some good items. Also, on Allmusic you can look up songs by title and see everyone who has performed it. In the case of “I’ll Fly Away” (linked above) it was composed by Albert Brumley, so you can see all of the artists who have performed it. If you find an artist who has performed several songs you like, you might give their albums a try. In the Sweet By-and-by has also been performed by a host of artists.
I can tell you right away that Johnny Cash is on both of those lists and is a great artist – however, what I like about gospel-country songs is often the “high lonesome” harmonies. You might do better looking for a compilation CD like the one by the Charlie Daniels Band if you want harmonies.
Search out the old field recordings made by Alan Lomax and released through Rounder Records. They’re readily purchasable on Amazon.
The Statler Brothers released an album (I have it on vinyl, I assume it’s available on CD) of spritual standards. My favorites from the album are “Power in the Blood” and “Turn Your Radio On.”
I recommend Rhonda & Sparky Rucker. I saw them perform at my college a few times in the early '90s, and bought an album or two: my favorite is Treasures & Tears. 
Bessie Jones and the Georgia Sea Island Singers did some great African American spirituals. Looking her up I found out she’s one of the Alan Lomax finds that Selkie mentioned.
She even does a rendition of “Oh, Death,” the song Ralph Stanley sings on O, Brother.
Check it out. Linky
The sequel CD to the O, Brother soundtrack is Down from the Mountain, and it has some on there, too.
There’s a great CD by Alison Krauss & the Cox Family called I Know Who Holds Tomorrow and it has that style of music, in addition to a rather hokey song. I’ll let you figure out which it is (I know it’s a traditional song, but it’s still hokey).
“Wade in the Water” by Eva Cassidy. One helluva voice taken from us way too soon.
More from Eva Cassidy: “Oh, Had I a Golden Thread”, “People Get Ready”, and “Wayfaring Stranger”. Hell, just get the Songbird CD.
The Best of the Hee Haw Gospel Quartet
Elvis did some pretty good gospel as well. Probably, not exactly what you’re looking for, though.