They used to be very common because they were imported. Now importation has stopped and there's hardly anyone who breeds them.
Try this website --->
Bird Breeder Search - BirdBreeders.com - Avian Directory of Available Birds and Bird Breeders
And search for "canary winged parakeet" another name they go by.
Some breeders that may be for sale
AvianBiotech.com - Find Birds for Sale
AvianBiotech.com - Find Birds for Sale
You might have better luck finding similar species such as the grey cheecked or golden winged brotos instead. We do have one member, Eliza, who has a golden winged!