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.
Limestone is not marble. You have two different chemical compositions building up, creating each type of stone. You also have two different processes making each stone itself. To create marble you need heat and pressure, making it a meta-morphic rock. Limestone on the other hand is a sedimentary rock and can be found usually under some kind of water.
Also, white chalkboard chalk is not Limestone either. It is made of compressed Talc.
Where I live I had a difficult time finding limestone until a friend of mine told me to go to a farming supply store or like tractor supply store. Where I live I know Fresno Ag had it and a store called "Tractor Supply." The one I bought was pelletized limeline and it's the same as granulated limestone. Hope this helps!
Longfeller, While true "limestone is not marble", marble is a form of limestone "metamorphised" such as dolomite. Though there are forms of dolomite limestone they are distinguishable from one another. Yet they are calcium carbonate or in the case of "marble" metamorphosed calcite or dolomit.
Also my friend chalk is in fact limestone, composed of mineral calcite. Or it can refer to magnesium silicate and calcium sulfate. To test your synopsis think of this, talc is not water soluble while chalk is. Talc is composed of hydrated magnesium silicate.