I'm not sure if this would be quiet or not, but here's an uchikomidai sold for kendo practice. Since it uses tyres as the target, I can't imagine it not being suitable for striking with a bokuto.
As for the mount in phitruong's post, I know that some karate dojos use those. It might be possible to get one from a karate supplier.
Hmm.. the top one looks like a tire from a gocart, interresting. Thanks for posting ! :-)