I think it is probably too late in the hatching season to get call duck eggs...maybe you could find some on Eggbid.com, but I doubt the fertility would be very good..most of the drakes are starting to molt.
I was able to find some earlier this week... got them set under a broody hen this morning. Hope I have a good hatch. You might try eBay. also There are currently 9 different call duck egg listings at the time of this posting.