Hi CaliforniaTropicals,
Check out your local Home Depot. I visited my local HD yesterday (Redlands, CA), it carries many cherimoya trees now. There are 4 varieties there: honey hart, el bumps, chaffey, and Vietnamese. The price is $30. In 2015, they will have 2 other varieties from La Verne which are Fino de Jete and Knight.
Simon,
The fruits you got from a Vietnamese Supermarket look like the fruits selling at the Vietnamese Supermarkets in Orange County. Probably they are not Vietnamese Cherimoya. My brother is a manager of a supermarket in Graden Grove, he told me that all the cherimoya fruits selling at the markets are imported from Mexico.