Maybe I should ask the LFS whether they order fish for people, and ask one of the employees to help me make a list of the smallest fish that they already have in stock.
it's best to talk to the owner or manager of the store, not any employee. most LPS get lists of available fish from their wholesalers each week, then they check off which fish they want. you need to review such a list if they'll let you, and choose accordingly. unfortunately, such lists do not use the scientific names we prefer using on forums, and they will often use common fish knicknames that are not even appropriate for the fish they sell. another LPS problem is they only sell the sex that displays best, so getting both sexes for breeding may be impossible. that's why you might be better off buying online especially now that winter is over. if you insist on special ordering locally; don't buy the fish as soon as they come into the LPS because many will die the first day or two after acclimation, so it's better they die in the LPS tank, then in yours after you paid for them. some LPS won't even special order certain fish due to their low survival rate from rough transport. it's also unlikely they will special order fish that they don't think others are likely to buy since they won't want to order the 10 fish you want, rather they'll want to order 50 that others might buy. try to learn each LPS local ordering cycle near you. If you call the store owner the day he usually orders, he's most likely to reluctantly add your request to the order list as any other day and he'll simply forget about it. even here in NYC where I'm a subway ride away from over a dozen good LPS, I still prefer ordering live stock online, and the only things I ever buy from them locally are heavy shipping items like gravel and rocks, and live food like blood worms, since most dry goods are marked up nearly 150% above online prices.
since your tank is only 5gal, you may want to look for nano fish that are very sociable with each other, not that they actually swim together in schools as you simply don't have much room for that. a good example of sociable tiny fish would be any of the pygmy cory species, or bumblebee goby's, even crystal red shrimp. some nano fish do best in species only tanks, so don't be afraid of a nano fish that does not community well, simply be sure to only get that one type of fish. The idea is to look for sociable fish that are generally bottom dwellers that like to hang out together, even with those of the opposite sex that might normally harass each other especially in such a small tank. you see, some nano fish go stir crazy in a small tank. take my 2cm long male Endler's for example. they love swimming together in current, and no tank is too large for them to constantly explore, but drop one female in with them, and they all go crazy. thus I would not recommend Endler's for a 5gal tank. actually, now that I think of it, getting all male display Guppys may not be a bad idea for your 5gal tank. they are not very active so a small tank is OK, they display well together, and most LPS will gladly stock them.
http://www.guppysa2z.com/thumbs/images/thumbnails.htm
guppys do crap a lot for such a small fish, so be sure to underfeed them.
However, if you insist on schooling activity that would work in a 5gal, the only fish that personally comes to mind would be Corydoras hastatus, which are the smallest 1-2cm of the pygmy corys that "flock" together swimming up and around, then down to cleaning the bottom as they explore your tank between taking naps on your plant leaves. unlike most corys, they don't have that annoying habit of having to gulp a breath of air at the surface every 30 minutes. they tolerate a wide temperature and pH range, and eat almost anything, however they can be sensitive during initial acclimation so buy a few extras. a dozen of those online would set you back around $60 with shipping, and don't even think of ordering them through an LPS. franksaquarium.com should have a fresh batch of them next week.
If you want something more colorful though less charismatic, then microrasboras would be the way to go.
