you can buy guide rings for regular sized rolling pins - try Bed Bath and Beyond or something like that. The problem with rings is if you need more that 16-18" of fondant the rings get in the way unless you have a really long rolling pin.
Some companies sell strips that you set on your counter and roll the fondant out in between them. I've seen them at Country Kitchen Sweet Art but you could probably make your own with leftover foamcoare.
I also use dowels lined up on the side of the fondant while rolling. I saw an episode of Good Eats where Alton Brown used rubber bands on each end of the rolling pins as guides. This lets you customize the thickness of the guide. I use a PVC pipe instead of a rolling pin. It's 18" long (and only $1.73!). It works really well with dowels.
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