Check with American Concrete Institute (ACI), they have some inexpensive ones with good documentation. However, if you build one yourself in Excel, you would understand the process much better.
If you have used Mathcad it can be very useful. Once the calculation is set up it would only require changes in the variables in the future. It also shows all of your work and can easily be checked.
While I am an advocate of MathCad I must say that a retaining wall complex or simple may be handled via spreadsheet quite quickly. I recommend a spreadsheet and a good text on foundation engineering...something like Braja Das or Bowles.
