TableCurve is great for the type of fitting you need, but it's not free... This is a good site where one can select operating system, rating etc. and look for curve fitting packages that are either free or shareware. There is a wide selection, so I am confident you will in high likelihood find what you need for free;
Good source of information will span over many many years. Recently, I wanted to obtain a curve fitting equation from the given graph image / published graph. I felt so powerless to do so. This CurveFit Tracer may be helpful to some of you.