I feel like you don't really need one. A guitar is only an octave higher and it can still cover a lot of the notes that a bass guitar does. Also, you could just use an eight-string guitar and tune the lowest string down to a low E. I know that some bands have eight string guitarists as well as bassists and the bassists play the same notes as the the eight string guitar players, but the bass will make the low end sound "fuller" because of how thick the strings are. But I don't think that really makes any much of a difference. I know this question might make me appear ignorant, but this is my own judgement.