A good place to find retired stamps by Stampin' Up is Etsy.com. I found the two stamp sets used below on Etsy, and even though the stamps were both used, they were in excellent condition and a lot less than buying them on Ebay.
SU Fifth Avenue Floral stamp -- retired and Autumn Splendor stamp for the leaf embossed in gold, also retired
Autumn Splendor embossed in bronze
Still needs a greeting stamped on it, but Autumn Splendor embossed in white