The factory 986 18' dimensions are:
F  7.5X18  50et
R  9X18  52et
It will be hard for you to find aftermarket wheels in those exact dimensions unless you want to go with 3 piece custom wheels like HRE (VERY expensive).   
 
Most aftermarket wheels are:
F  8X18  50et
R  10X18  47et
These work but you'll need to pin your e-brake cable out of the way or it will rub.