Somewhere -here on this very site- there was a similar thread, and it had a link to a really cool place online that has FREE SHEET MUSIC of stuff that's in the public domain.
I don't have the time to look now, but if you do a search, you can find it. Search for Bach.
I found it, and downloaded all 6 of his solo Cello suites. They're perfect for 6-string bass -same range, just played an octave lower. I'm currently working on the first prelude of suite #1, and have almost finished it. Good stuff.
And there is PLENTY more stuff there. You can seach by composer and/or instrument. If I have time later, I'll try to find the link, but again, you can probably do it with the search function.