They should come with your rims. If not, you may have to pay for it. I think they are like $5 each.
I never knew there was such a thing . . . so if I bought aftermarket I should get them with the rims?
It really depends on the rims, I guess. I use Racing Harts exclusively. My M5's come with 2. I called Dazzmotorsports and they told me I only needed two for the front. When I got the Z5000, two came with those also.
They look like this: