I have spent lots of money at "the fish lady" (Lagoon). All of the corals I have bought at the petbox havent been really much of anything. Not sure why, I just havent had any luck with there corals. Fish, yes, I have had good luck with there fish, and I have also bought fish from the fish lady.
Here are 2 stores I have bought from that arent listed:
THe store in Naha. (I just went there last weekend for the first time) Its kinda hard to tell you where its at, but here it goes. From Sports Depot parking lot in Naha, turn left (go away from 58) go all the way until you hit 329. The road will T at 329. Go right. Get in the left lane and go all the way until you hit a family mart on the left. Pull in there, and park, then the fish store is on the other side of the road about 50yards down. You will see the lights from the outside. Its just like the lagoon but a much bigger selection of corals. They are all taken from the ocean, but good corals. Also, they are a little cheaper than the lagoon. Its gonna be my new main source for corals. The lady there didn't speak english very well, but she wrote prices down and if we had questions, she could understand us, but didnt know how to speak back to us, so she whipped out her pocket translator.
Another way to get there, take 329 all the way to naha. When you go under the mono-rail get in the left lane. You will go through the intersection FROM sports depot. From there, just follow the directions above FROM where you turn right on 329.
For the other place:
Its a coral farm. Go out gate 2 until you hit 329. turn LEFT on 329 and go up the hill...it will be on your left. It will sneak up on you so you gotta watch out for it. Its a VERY nice store with very nice corals, but your gonna pay. $$. go to there website
http://www.ppmokinawa.com find what you want, and print it. then take it in there. I google the web address, then google translates it to an extent where I can understand it. This place is closed on Wensdays and open till 8pm every other day.
ALSO, there is a cafe called "goby cafe and marine aquarium" in american village. I highly recommend this place if you are into corals. Go down by the ferris wheel, look at the ferris wheel and turn around and look up, you will see it. If you go there around 7:30pm you will see the MH lighting. He charges a 300y cover charge if you are going in there to eat, but if you get there right at 8, and just tell him you are there to check out the corals, he will let you in and check them out. This restraunt is in conjunction with the coral farm listed above.
I have been told of other fish stores on island, but these listed above are suspossed to be the best. I have also been told to go out at low tide and you can find your own corals. I have been out 3 times during low tide and havent found anything.

I am still searching though.