Car parts vary drastically in price from place to place and whether or not you want to buy new or used. Genuine parts are usually a lot more expensive than non-genuine parts but some people say that they are better quality and therefore last longer.
If you take your car in to a garage to have repair work done, they may not offer you the choice of genuine or non-genuine parts, so always be prepared to ask. If you are taking your car to a main dealer and the car is under warranty then they will probably have to fit genuine parts in most cases.
If you are happy to buy second hand parts then try and buy from somewhere you will get a warranty from or at least some sort of guarantee that it is in full working order when you purchase it. There are a number of websites where you can buy second hand car parts. Some are sold on sites such as Ebay and Gumtree but there are also specialist sites which allow you to search for parts by make and model or your reg number.