MetaTrader 4:
MtApi installation file: MtApi_Setup.exe (ver. 1.0.40, updated 13.02.2018).

MetaTrader 5:
MtApi5 installation file (32 bit): MtApi5_Setup_x86.exe (ver. 1.0.17, updated 02.03.2018).
MtApi5 installation file (64 bit): MtApi5_Setup_x64.exe (ver. 1.0.17, updated 02.03.2018).

Source code:
Source code of MtApi project is available on GitHub. It has the MIT License and opens for using in commercial projects.