I have seen Cubitainers ocassionally at REI. Water bottles work almost as well.
There are a few places on Oahu that have lots of stomatopods. Probably the best is Kaneohe Bay where I do most of my collecting. Assuming that you can't get onto Coconut Island, go out at a low (lower the better) tide and walk slowly in water that is from 1 to 6 inches deep. I usually try to time it so that I am walking the water's edge on the incoming tide. As the water covers Pseudosquilla ciliata burrows, they come out to forage on small fish and shrimp before the water gets deep enough to allow large fish predators to move into the area. This is the best time to spot them. Generally, they will dive for cover as you approach, but with a net and tickle stick, you can usually chase them around until they tire and you can ge them in a net. If they go down their burrow, you can usually force them out by sticking your finger into one end and forcing your hand/finger through the burrow until the animal pops out the other end. Usually you can see muck coming out of the second entrance. If you can identify the second entrance, put your net over it and the animal will flee right into it.
There are also Gonodactylaceus mutatus in this same area, but they generally stay in pieces of rubble. Try picking up porous, grapefruit pieces of coral rubble (many will be covered with clumps of algae) and inspect them for perfectly round holes about the size of a dime. If the cavity has an entrance that is perfectly round and smooth, there is probably a stomatopod inside. Probe the cavity with the tickle stick and if you feel a strike, there is an animal inside. Since you should not smash the rock, you will have to try to harrass the animal until it bails. Unless it is a female with eggs, that usually is fairly quickly. (I usually take a bucket so that I can do this without losing the animal in the water.)
You can also snorkel for P. ciliata, but you will see far fewer.
If you are a diver, I can give you directions on how to catch O. brevirostris, one of the greatest stomatoods going!