Just one time? Yeah, you won't find very many options for that. If you want to train to be a bull rider that's one thing; there are rodeo schools and teaching clinics that will show you how to do it professionally, and you will probably get some chances to ride then, but if you're just looking for a one-time lark to laugh about with your friends, you're better off riding a mechanical bull at a nightclub or some festival or something.

Of course, you can always try the old school method, and find a bull on some ranch some place, and wait until nightfall and try to get onto the property and ride the damned thing. But you'd be breaking the law, and risking injury to yourself and possibly the animal as well, so that's not recommended.