Costco uses nitrogen to fill tires at their tire centers. If you're a member, they'll probably do your bike tires for free.
Sightglass has an excellent point though. Checking your tire pressure regularly may show a problem before you come out and find a flat tire. If you do it on a regular basis, you'll know how often you have to add air. If it becomes more frequent, look for the reason.
Nitrogen is also moisture free, so if you live in a humid climate you are adding a lot of water vapor with your air. As the smaller molecules seep through the rubber, the water stays. You add more air, and the water content inside your tire increases each time. You can see where this is going. I've seen car tires with a cup of water sloshing around inside.
So the old joke about changing the air in your tires has some validity. It's not usually a problem here, but I have a drier on my compressor anyway.
