I've seen plumbers use toilet pluggers in sinks and bathrooms as well. On the sink if you have two drains, put the stopper in the other drain. The plumbers around here use this style, which I have and they work great, only $6 at Walmart. You might also take the allen wrench that came with your disposal and turn it a few times from the bottom in case it's slightly locked up.
Master Plunger with Air Release Valve - Walmart.com - Walmart.com
Allen wrench