to my knowledge, no one makes one for a bicycle. unless you wanna hang it from your seat, or spend a ton of money for a motorcycle one that you have to modify, the only other option is to make your own.
you can make two "L" brackets and mount one to the rear axle and one to the frame for a sideways mount, or copy something like this: Welcome to accufast2d.net
in california, your plate can be sideways, but it has to be visible from the rear. my buddy had his ziptied to the side of the frame and a cop told him that he could get a ticket (he didn't get one, though...)
the plates really are a pain. they're way to big, and even if you do come up with a good way to mount them, it never matches the bike.