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.