especially for you Jesse! Sorry for taking so long to reply - work got in the way!
The tour is widely advertised on billboards around Ubud for 360,000 rph, but we bargained down a little. You will be able to buy tickets where it is advertised (could be a little stall down the side of a shop, like where we got ours, or even a material shop!). No need to contact anyone in particular. Don't worry that they don't ring to see if there any vacancies - they WILL fit you in.
Its great that you are all used to bikes but DON'T rush it. Take your time and LAP UP the scenery, its NOT a race. I couldn't resist putting my 2 cents worth in about this. Last trip there was a cyclist that was obviously quite experienced. Trouble is I don't know that he got the point of the trip because he was too busy showing off. Even his girlfriend had no sympathy for him when he came a cropper trying to beat the rest of us!