ebay?
link to my search
several buy-it-nows for copepods.
But the fact that you're posting this in the nano section scares me... I wouldn't keep a Mandarin in less than a 75, with tons of rocks. I love the things, they're beautiful fish, but their specialized diet makes them candidates to starve to death without the proper tank setup.
If you're keeping a Mandarin in a small tank, my first suggestion is trade it in. For his sake. If not that, then copepod supplementation is a definite, absolute must. If you think he's not getting enough to eat, you're probably correct. Be prepared to be buying copepods for as long as it lives.