If they had them in once, they can get them again. Let them know you're interested, and ask them to reserve a few for you. While you're at it, check what species they're getting. Halfbeaks do tend to be irregular because they're wild caught, so rely mostly on shipments from SE Asia. So when you see 'em, catch 'em.
Do be careful not to confine too many males in one space: they fight. Also, big ones can be predatory, so don't mix them with anything very small (like Endler guppies). On the other hand, halfbeaks are great fish, and well worth the extra effort. Once you get your first batch of babies, you'll get hooked on "rare livebearers" and you'll wonder why people get all fussed over platies and guppies when they could be breeding the rare stuff instead!
Cheers, Neale