ATV are not that much common in the market. That is mostly because they are banned in some states of countries like Canada, Australia and United States. Another reason is that they are too much accidental; another blow is that people do not wear helmets while riding it. You might find it hard to find a guide for ATV parts over the internet. What you would find over the internet is a huge market selling parts for ATV. This is where you should look for parts; do not go for a dealer.
One thing which you need to keep in mind is that never purchase performance parts second hand. If it is a body part like view mirrors or lights then surely purchase them second hand. If you have a car and a damaged door then purchase a second hand door and get it painted. This would save you a lot of money. ATV parts which need your real attention are shocks and stuff. These things help in your ATV to run. It would be best that you purchase them first hand from the internet. Things like a brake shoe wear out after some time. Same goes for tires and filters. You could go and purchase these ATV parts over a whole sale shop. This way you could store these parts and get them on discounted prices as well.