Actually, it has been listed in here (https://charmm-gui.org/?doc=archive&lib=csml) with document named as toppar_all36_moreions.str but i could not find it. If anyone has, could you please share with me?
I didn't find the parameters for the phosphate ion on that website either, however you can get them using the CGenFF software (https://cgenff.umaryland.edu/). I have attached the parameter file obtained. I hope it helps you.