Not all slope stability software is real expensive. Depends on what you want to do with it.
ReSSA by Adama Software (US$1150)
G-Slope by Mitre Software (US$1250)
Geo5 by Fine Software (US$750)
Geo-Tec B by Interstudio (US$500)
New one I just found, MacSlope, for Mac and iPad ($700)
You can sometimes find free versions of older software such as ReSSA 1.0, UTexas2 or Stable. Some software have different demo or rental versions that can help learn the software at minimal expense.