Most choke manufactures make the 20 gauge chokes and should have the winmoke system. However most sportstores don't cary many of them. I knows that Cabelas and basspro carry a few as well as wingsupply. Check out hastings, H.S. Undertaker and comp-n-choke.

My daughter has had very good patterns with her 20 gauge 870 with the hastings choke.

Good Luck!