Most sources of information say that since the natural pollinators don't live in the US, you need to hand pollinate the flowers with a small paintbrush. This is not always true. I know many people who don't do anything to pollinate the flowers and still get a lot of fruit. The fruit can be hard to get. Cherimoyas grown in California rarely leave the state because of the high demand.
Farmers markets are probably the best sources in areas that it is grown.
Mexican and Asian markets are also good sources. Health food stores will occasionally have them. You also might try looking for a source on the Internet. The first thing to look for is any damage to the cherimoya. Don't get me wrong, I am not one of the spoiled Americans under the delusion that produce should be totally devoid of any blemishes.