How can Dark and Silver Super Sonic be unlocked in Sonic R

If you're speaking of "Sonic R" as in, the old Sonic racing game (http://en.wikipedia.org/wiki/Sonic_R) then you cannot get "Dark" or "Silver Super Sonic".

You can get: Sonic, Knuckles, Tails, Amy, Metal Sonic, Tails Doll, Metal Knuckles, Robotnik ("Eggman") and Super Sonic.