Go to an online Hip Hop record store. An online store that specializes in Hip Hop is more likely to carry the artists that you are looking for. Because it caters to the Hip Hop community, it will carry a wide selection of vinyl as well as CDs and digital music. Places like 360Vinyl.com and Undergroundhiphop.com are good places to start.
Go directly to the record label. If you know what label an artist is signed to, you can go directly to the label's website. A lot of labels make vinyl records available on their online shops.
Look on Amazon.com or Ebay.com. Amazon and Ebay are two of the most popular stores on the web. They stock a wide variety of merchandise for sale by individuals and companies, including vinyl records. Use the search features on the websites to locate vinyl records for sale on the sites.