could any of you how are considered self taught can tell me how did u learn bass theory, i have been playing bass for about 18 months and my technique is okay but i reallyhave to start learning some theory as i have trouble playing the basslines that i think of
There is no such thing as "bass theory". It's the same theory for all instruments.

That being said, most guitar stores have a couple of books on music theory for sale. Your local library probably has some too.
A good starter book is "The Complete Idiots Guide to Music Theory". Its fairly comprehensive and while its not bass specific, its great when you start testing the waters. When you want to get serious--the AB guides to music theory are the way to go.
I learnt everything my self with a little help from guitar pro. I'm playing the bass for three months and I can learn one song in two days, guitar pro really helpt me with that.