While not as popular as other styles there is a definitate place for the 6-string electric bass guitar and So Music can offer these. The 6-string bass is usually tuned like a 4-string bass but with an additional low B-string and an additional high C-string, but there are a lot of tuning variations available, including matching the tuning of an electric guitar.
Danelectro are cited as being the first producer of a 6-string electric bass back in 1958 when they released the Longhorn6, but Jackson Guitars Anthony Jackson were probably responsible for the modern 6-string bass which eveolved from their original Contrabass guitar.