I'm having trouble finding stores that have new copies of Pokemon Black in stock, so Black 2 might be easier to find new. Or you could just buy it off Amazon.
If you're willing to buy used, you could probably pick up Pokemon Black used at a local used game shop. Unfortunately for us, Pokemon games their value extremely well and GameStop (which wants an absurd $33 for a used copy) put most one-shot used game stores out of business.