Helmet choices are pretty varied. Some like to wear a regular motocross helmet, some like to wear a full face race helmet, and then Fly makes a pretty cool hybrid that is a motocross helmet with a full shield that you can use, or you can keep the shield up and use goggles. There are also open face helmets that provide minimum protection, and some like to run those as well. I personally prefer the Fly hybrid helmet or my full face race helmet. I don't like dirt and sand in my teeth - lol!
As far as where to buy, you should probably go to a dealer or shop that has them in stock and try them on to be sure you get the right size. Sizes can vary A LOT per brand......and you will want your helmet to be comfortable and fit you right. Even if you just try on helmets and find one you like, but the dealer is pricey, you can then go on line and find it at a better price, but at least you will know exactly which helmet you want.
this is what we use and its awsome we used to use over the wall helmets but too much work putting on helmet then head set annd goggles now slip on helmet and go and now fresh air pumped in blows out dust dirt and sand
x2 on the dual sport helmets. Only downfall is they are HOT in the summer even with all the vents open. I usually take my dual sport and moto helmet with me on every trip and just wear what's comfortable that day.
Well the only thing I could add to all the good advice already given is if you go with a mx type helmet and are going to use goggles try on the different goggles with the helmet that fits you perfect do to the front of some helmets has different cut out size and some goggles are to big in size and will push down on your nose to much like shoei helmets need small goggles to fit comfortable.
Try to get a helmet that is comfortable first off. You will be wearing it all day some times. Start by trying on different styles you may like and wear them around for at least 10 minutes in the store. When you remove it, look for pressure points on your forehead and cheeks. Did it leave you red?
Try to get the lightest helmet you can afford, stay away from modular unless you are a linebacker, those puppies can weigh 6 lbs!
this will lead to all kinds of neck pain and discomfort.
Remember that your head is the most important thing in the car, don't skimp on lil biker helmets so you can drink beer while you ride, that's what straws are for