Yes you can buy ammo online in NY until Jauary 15, 2014
I received an email from Midway USA, part of which I copied below.
Now that we've had time to read through the law several times and have received some answers from the NY State Police, we can fix our mistakes. Effective immediately, we've lifted our newly imposed New York restrictions on ammunition, reloading components and magazines with a capacity of 8 to 10-rounds. Now, unfortunately, we'll have to reinstate these when each portion of the law goes into effect (April 15, 2013 for magazines and January 15, 2014 for ammo). And, we'll have to reinstate these restrictions about 2-weeks prior to the effective dates to make sure Customers have these products prior to those dates (clarified by an attorney for the NY State Police).