There exists a bunch of population genetics forward and backward (coalescence) simulation platforms. Here is a non-exhaustive list. They all differ and you'll have to go through their manual to see what is more adapted to your needs.
List of softwares
Here is a long list of such platforms. The list might arguably be a little bit outdated today though.
Most common softwares to my experience
Some are more known than others. Personally, I already saw uses of the following platforms in publication: SimCoal, Nemo, Slim and SFS_Code.
My own software - SimBit
Of course, I must indicate my own simulation platform SimBit.
My experience
I have personally used NEMO and SFS_CODE in the past (I now use SIMBIT only). So I can only talk about these 3 below
Comparison between Nemo, SFS_code and SimBit
All three models are well updated and maintained by their authors and of course everyone has a access to the source code.
Performance
SFS_code is very fast for small simulations. The issue of SFS_code is that the RAM usage and run time are exponential function of the genetic diversity. This means that some simulations can quickly become totally unmanageable. But it is a great tool if you expect little genetic diversity.
NEMO is slow in comparison to SimBit and SFS_code but is very flexible.
SimBit is very fast (but slower than SFS_code for simulations with little genetic diversity) and more flexible than SFS_code. However, SimBit is not as flexible as Nemo typically when it comes to the order of life-cycle events (is migration before or after selection for example).
Hope that helps. Good luck!