Dragon fruit can be found year-round, though the season peaks in summer and lasts into early autumn. Its gnarly looking skin makes it easy to spot in any produce market, but not every grocery store will carry dragon fruit. If your local supermarket does not stock it, check out a specialty store or Asian market.
To choose a ripe dragon fruit, look for bright, even-colored skin. A few blemishes on the skin are normal, but if the fruit has a lot of blotches, it may be too ripe. Hold the dragon fruit in your palm and try pressing the skin with your thumb or fingers. It should give a little but shouldn't be too soft or mushy. If it's very firm, it will need to ripen for a few days.

Technically, the types of dragon fruit can be broken down by genus/species/hybrid. The genus Hylocereus has three different species of dragon fruit, while Selenicereus has one, and each species features several hybrids. However, it is much simpler to categorize dragon fruit by their skin and flesh color.
Pink-skinned, white-fleshed dragon fruit (Hylocereus undatus) is the most common. This is the least sweet variety and may be sold under names such as Alice, Cosmic Charlie, David Bowie, Guyute, Harpua, L.A. Woman, Neitzel, Seoul Kitchen, Thompson, and Vietnamese Jaina.
Hylocereus polyrhizus has pink skin with red or pink flesh and is often larger and sweeter than the more common variety. Look for Bloody Mary, Red Jaina, Voodoo Child, and Zamorano.
Hylocereus guatemalensis is the third species of fruit in this genus. It has bright pink skin and purple flesh. Native to Central and South America, it's also grown in Florida and marketed as American Beauty.
The smallest and sweetest variety has yellow skin and white flesh. Grown mostly in South America, Selenicereus megalanthus is the most difficult to find as it is not mass-produced.