there are many great sites for learning. Free as well.
You Tube, if you take the time to really navigate it, has some phenom lessons.
www.activebass.com is a very good site with excellent intermediate to advanced lessons. Some of the posted beginner lessons are good, too.
www.studybass.com is very good for intermediate bassists.
www.playbassnow.com is a GREAT SITE if you are disciplined enough to stick with those lessons. That guy "MarloweDK" is a wonderful player and very generous to post all his daily lessons. If you force yourself to learn what he's teaching (his stuff is challenging, but doable in time), you WILL become a very good bassist.
Happy Hunting!