The mapping tonal harmony app for iPad is a pretty good tool, I think it cost about £8 or so. Scott's Bass Lessons, and videobasslessons.tv are really good learning communities but do cost quite a lot at about 40 dollars a month each. If you are willing to shell out that amount of cash I would recommend getting some proper face to face tuition, and read as much as you can. You will be surprised how quickly you will pick it up once you start reading every day with your bass on your lap. The mistake I made when doing what you are trying to do was I tried learning theory and reading as a separate thing to my playing. Have your bass in front of you as much as you can so that you relate everything to the bass.