As of 2015, RockAuto.com is one source of affordable parts for a 1999 Cherokee. It has parts specific to that vehicle as well as universal parts, such as oil and air filters. For vehicle-specific parts, it offers a choice between original equipment manufacturer quality or less expensive aftermarket quality.
Amazon.com and eBay are two additional sources for replacement parts if the exact part needed is known. For example, searching for an alternator reveals how many parts are available and also provides compatibility information. The ability to easily compare prices is one advantage to using either of these sites for pricing purposes.