Opuntia subulata cristata
(This plant comes in a 3.5" pot. You will receive a very similar plant to the one in the picture. It may or may not be blooming at the time of your purchase. The plant is shipped in its pot to prevent any damage to the roots).
Opuntia subulata cristata is a very popular cristate form with undulating green fans. Member of the family known as Prickly Pear. Opuntia subulata is normally columnar with slender stems to 6' or more.
Some of the information in this description has been found at desert-tropicals.com, llifle.com and cactus-art.biz