None of the killiefish are easily available at retail stores. I've only seen one carry anything other than Apolicheilus (sp? - Golden Wonder) killies. That said, there has to be an aquarium society or two in NYC, possibly even in queens, and likely meeting somewhere near a subway stop. Fp. gardneri is one of the more commonly traded ones, as are some of the aphyosemions and epiplatys and some of the other fundulopanchaxes. If you don't have the wherewithal or desire to culture your own live food, I'd be very certain that you got a species that will thrive on prepared food (such as Fp. gardneri), as more than a few killies do poorly on anything that's not live.
Many killies, Fundulopanchax spp. included, will, under the right conditions, lay their eggs in a peat substrate, which can then be removed and kep just moist for quite some time, and the eggs will remain viable and hatch when put in soft water. So your other option is to purchase eggs by mail. They can be sent US post without much risk of them dying, so shipping is not exhorbitant like it tends to be with live fish. I believe that there is a section of aquabid.com for nothing but killie eggs. You should check it out. (Incidentally, if you have the space for them, the Blue Gularis is a great fish too.)
Regardless, if you are at all interested in breeding fish, I cannot recommend highy enough that you find a local aquarium society to join up with. Aquarists are a really odd assortment of folks, and can be a great deal of fun as well as an invaluable resource.
I used to find varieties of Aphys., Epip., Fp, and other Sp in lfs which specialized such oddballs.unfortuntely gone about 6 yrs ago. I am done with breeding fish for now and I have not tempted to breed killies since 80's.Well, that doesnot mean I have not recovered any eggs when Species tanks were running.
The reason for asking their availablity was to see if you know of any reliable resources online where members can have easy access to killies.