I'm sure almost any gym has an 8 stationer like this one.
I get an ankle attachment, attach it to one of the inner attachments under the pull up bar, bring the adjuster all the way down, attach it to my ankle.
Then I use about 80-100lbs resistance, grab the post on the opposite end and just throw knees.
http://www.fitness909.com/images/Jungle_8_Station.jpg