mikey, I've been to every major petstore in the 5-boros and they all disappoint when it comes to planted tanks and nano needs. since it's a small tank, you are much better off buying from ebaY where tons of rocks are being sold each day which is what I will end up doing once I get a larger tank. first research what types you like, make sure to research that they are inert, then go for it. try to pick a seller who flat USPS ships for $8-10 as they are usually the most honest and best to deal with. don't think buying locally will save you money as most LPS know they are competing with online sources so they build the shipping into their already exorbitant markup.
if you insist on shopping locally so you can see and touch the rocks first, a good place would be Pacific Aquarium 46 Delancey St (212) 995-5895. call them at least twice (different guys will give you different stories) to be sure you come right after their bi-weekly rock&wood delivery. everything is displayed dry, but they always have a few empty poorly lit tanks you can use, to see how the rocks look immersed.
a better place that might be worth driving out to would be Fish Town USA, 196-33 Northern Blvd (718)-539-5396. also call first so you get to arrive after a fresh rock delivery. keep in mind this places inventory is half salt, half fresh, so alot of their rocks are for salt and brackish tanks. this place is also one of the few great, algae free, aquatic plant sellers around New York, but they are mostly large leaf low tech plants.
I've never done this, but always wanted to try, so let us know how it works out;
most of the Flower Market District is still around 27th Street and 7th Avenue. there are a few Bonsai hobby supply houses on the street and upstairs. I imagine most of them sell wonderful nano appropriate rocks at wholesale prices. so if you work or commute through that area, it may be worth the pit stop. try to go before work or during your lunch hour while the place is still buzzing, since after 3pm most of the area shuts down for the day.
one last piece of advice when rock shopping; alway buy a few more pieces than you need. that way when you get home you can try a few extra combinations in your tank. so you waste a few dollars on rocks you don't need; sure beats having to go back for more
and remember; for iwagumi you only need 3 rocks, so choose them wisely, grasshopper...
here is the type of rocks most local pet stores will sell;
Natural Aquarium Rock & Stone
so if you want something more exotic, you MUST buy online.
here are some rock type ideas to explore;
Feller Stone - Welcome to Feller Stone
rocks I'm exploring for my own tanks;
petrified wood rock (especially from Arizona)