Welcome!
Nice looking quad!
The first step is to Google a chain chart, and measure your chain to determine what you have; my guess is 428. Once you've figured that out, you can buy a HD version from
www.bikebandit.com,
www.rockymountainatvmc.com or
www.motorcyclesuperstore.com. You can search each site, and you can even ask them to match prices. I like RK or DID chains.