Pomacea diffusa should be readily available even online. Try aquariacentral.com, our sister site if you are willing to have them shipped although some may simply be near your area and you could just pick them up.
If you are close to NYC there are a couple of friend LFS that will help you get your exact size request and etc. Or like mention by someone earlier try to ask your LFS to order... even the Petco around where I live do orders for me.