I purchased a tubastrea sun from this site and have had great luck. It is a wonderful addition to my tank, in fact if you can create a cave with the live rock in your tank place the sun under/inside it. They tend to come out only in the dark. After two months, however, mine are out and ready to eat throughout the day when the light is on!
You may need to start slowly by coaxing them out with brine shrimp, once their tentacles are showing they can grab the food. They will look bloated if healthy.
They are a fascinating addition to any tank.