Ah, we buy natural peanut butter and use a chopstick or table knife, but I guess it depends on the size you buy? Ours comes in just larger than pint jars. If you are getting something similar to the Costco size jar, you'll need something slim and metal like a meat fork--that's what we used to use when we bought the Costco "Adams" brand natural peanut butter. Those jars were huge!
Or, if you have a hand-held mixer you can use ONE of the beaters--we did that a few times as well, you just have to scoop the more solid stuff up from the bottom first, then mix.