Motocycle shops of course.
Price depends on what you want quality wise. The cheapest helmet in the shop will be just as safe as the most expensive, but it will end up having a different fit/feel to it.
HJC helmets - bottom of the barrel in the helmet world, good fit (at least on my head, so I got one), but still going to set you back $150 or so for a half decent full face
http://www.hjchelmets.com/
Arai - argued the best helmet made to date. They are amazingly comfortable, and completely adjustable. I didn't get one simply because they fit people with more round heads which mine isn't. Cost ranges for about $250-$700 depending how nice you want.
http://www.araiamericas.com/home.html
Shoei - also a quite good helmet, but also has a very specific shape to it. This actually hurts my head, but to others it fits like a glove. If you don't have a very oval shaped head it probably won't fit you right. Costs about $250-$500.
http://www.shoei-helmets.com/
There are other manufacturers, but those are the most common you will probably see at your local bike shop. I personally would just grab a halfway decent HJC, simply for the price. The fit my head fine, and cost half as much as an average higher end helmet. The thing you have to think about is the fact that you aren't going to be using it much. If I still had my motorcycle and was riding all the time, I would buy an Arai hands down.
- Ryan