Trying to conceive but have tubes tied.

I have had my tibes tied for 10 years now. I have heard so many stories abour women having kids after this. I have been trying to conceive for 3 years and cant seem to get pregnant. I have 3 kids 2girls 1boy my fiancée has 2girls we want another boy. Anyone know of any ways of success? We cant afford to untie them and we surely cant afford inveto. Please help.