I know it ruins the magic a little, but you can go to most aquatic-y places (fish shops? am I being really dim in that I can't think of a proper name? ) and ask for brine shrimp- exactly what Seamonkeys are, and very commonly available.
I had some when I was at primary school. I bought them some 'sea-diamonds' which apparently the sea monkeys were supposed to toss around like beach balls and even surf on (which would give them vital exercise). Needless to say, I was disappointed. The banana flavoured sea monkey dessert was a bit of a rip-off too. I want some more monkeys now I've seen this thread. My last lot all had names from Lord of the Rings. Or the ones big enough to be visible did.