eBay. Heads up, you need a 2.75" flange, not 3". Most of the ones I saw were 1/2" thick which is what I got.
Here is what I got:
http://www.ebay.com/itm/192149028997...%3AMEBIDX%3AIT
You could most likely get one for cheaper. I needed mine quick and this seller had the quickest estimated arrival at the time.
Double post fail