you can buy them from a dealer or steal them from a friend.
my friend side cliped a telephone pole in his 2000 silverado, and his passenger mirror fell off. So he had to get a whole new mirror, which came with a new chrome cap. So he gave me the one off of the mirror that fell off... no scratches or anything.. just like new. And then i bought the driver's side from the dealer for pretty cheap.
i just got a flathead screw driver and popped off the black plastic covers. They just pop in and out of 4 grooves. Just get the flathead under the side and pry it off. And then press the chrome ones in, being careful not to bend them. It took me 2 minutes to do.