Depends on how versatile you want it to be. If you want basic shapes, or the ability to add flat plates to reinforce an I shaped section, you can get CSISections for your phone (they have a suite of free apps that are pretty handy). If you want something more powerful, that can do custom fabricated shapes, extruded shapes, or a built up shape comprised of several standard shapes, then you may need to pay for it. I'm working my way through the SkyCiv trial right now, and they have one. Haven't used it much but it may be worth a try.
If you don't have to use them very much, though, it's probably worthwhile to do it by hand. Either that, or use the REGION, MASSPROP method in AutoCAD. It doesn't output the plastic section modulus directly, but you can locate the PNA pretty easily and then find the centroids of each half. Then it's a simple moment calculation.