Limestone, calcium carbonate, is everywhere.  If what you want to do is examine the effects of acid on limestone, you can use just about any form of limestone.  The damaging effect of acid rain on limestone sculptures and buildings takes a lot longer than a science project timeframe, but you can accelerate it by using stronger acid.  You probably had this figured out already.  I believe that "acid rain" refers to very weak hydrosulfuric acid dissolved in rain water but any form of acid should do.
 
I would look up a marble dealer in your area.  Marble is limestone.  Where I live, there are several places that sell huge slabs of marble to be cut up into kitchen counter tops and they always have scraps.  Make sure, however (by talking to someone at the store who knows) that you are getting real marble, not granite.  I've been tricked by this - some forms of granite look just like marble.
 
White blackboard chalk is also (usually) make of limestone.  It is softer, but the effect of acid on it is the same.  Wallboard may contain limestone, but I think it is usually gypsum, which is a different compound.  Nurseries (the garden type) may have limestone construction stones as well.  
 
I'm less sure of where to get acid in your area.  Vinegar may work.  with the right equipment (glass bottle and glass eyedropper and safety glasses always) I would try to get some hydrochloric acid (HCl) from a gas station - one that has service bays and especially one that sells car batteries, since HCl is used in car batteries.  Remember this basic lesson from chemistry class: when diluting acid with water ALWAYS add small quantities of acid to larger quantities of water, not the other way around!
 
I also did a web search on "limestone Manhattan" and "limestone Brooklyn" and it came up with a lot of hits.