So, basically, as long as you have the mission to kill them you can get at least one racing certificate for one raider car killed. Here I will detail the simple method of getting as many certificates as you want, with this method you could easily get hundreds in no time.
1) Get the mission, all you have to do is talk to Sally or Brick.
2) Leave the city and go fight at least one raider, it's not hard as they pounce you soon after leaving the city anyways.
3) Use the mega item option of the trainer, this will convert the bounty item in your inventory with a picture of a skull from whatever number you have, to 99 like any other item in your inventory.
4) Just walk up to the bartender to redeem your reward.
5) Repeat as often as necessary!
I hope this helps all of you, some of you probably figured this out without my help but at least now this is in a thread by itself where it can't be lost in posts.
Thanks for posting this find.