Okay, I called both the store here in VA and the store in NYC to compare stories. VA said any B&N or BN.com receipt will get you a wristband. The guy at the NY store said that if you buy the book at the store, you get a wristband (as supplies last). You CAN bring in a receipt from another B&N and get a wristband, but those buying the book at the location get priority, so there may not be wristbands left.
Basically, show up as early as you can to get a wristband, whether your receipt is from the NY B&N, another B&N, or BN.com.