Have you noticed green caps on the tire valve stems on some of the cars lately? Green cap mean it is filled with nitrogen instead of air. It is true that pure nitrogen doesn't leak out as quickly as regular air because a nitrogen molecule is larger than an oxygen molecule. And, for this reason, tires stay fully inflated longer.
The catch is that topping off the nitrogen typically costs about $5 to $10 per tire and the initial charge for filling them can be much higher.
One thing to consider, though, is that only about 93 to 95 percent of what is inside a tire is nitrogen and the rest is air even when it is filled with nitrogen, whereas ordinary air consists of 78 percent nitrogen. The difference is not that significant! And, regular, compressed air is a lot cheaper and still free in some places.