As of 2015 HO scale slot cars are available for purchase online from hobby sites such as AutoWorldStore.com and Hobbylinc.com. EBay.com is also a good source for HO scale slot cars.
There are many different types and manufacturers of HO scale slot cars, so it important to shop around. Hobbylinc sells AFX brand slot cars, while Auto World sells its own brand, called AW 4Gear. Although prices are similar, there is a big difference in styles and colors. For example, AW 4Gear brand includes not only racing cars but also street vehicles and semi trucks called racing rigs.
A greater variety of slot car brands and prices are available on eBay. There, brands such as TYCO, AMT and AWK are available for purchase as well as AFX and AW 4Gear. Each of these sources also sell parts and accessories for HO scale slot cars.