such as multiplex ip, we can find it in install directory.
our end customer would not like to use qsys or platform designner
how can we get these ipcores source code?
If it's not a licensed, encrypted IP, the code is available in the location where you generate the system, by default, a folder named after the .qsys file.
If you want the HDL, you can generate HDL code for IP according you what is desired. Once finished you can start from there, I am not sure if there is other alternative than generating HDL file for IP method.