You can find off road motorbikes for sale online from the manufacturers' websites and online stores. You can also buy off road bikes from approved dealers online or from online stores' automotive departments such as eBay.com. These bikes can either be new or second hand.
Apart from online stores, there are local dealers with showrooms and yards who sell motorbikes. While off road bike manufacturers' showrooms are a good place to start, there are independent motorcycle shops that also offer second hand bikes. Websites such as Craigslist and eBay have independent listings from people who want to sell off their off-road motorbikes.