There are several different ways of buying car parts, and which one you choose depends partly on which part it is which needs to be replaced, and, not least of all, your budget.
If money is no object and you want the very best products than buying new parts is the obvious option but this can be a somewhat uncertain method; are you sure that the part that you are buying comes from the original manufacturer of your vehicle? If not, was it manufactured by a company that supplies the manufacturer? There are a lot of companies producing spares for cars, particularly the most popular models, and whilst many of them are of first-class quality some of them are not quite up to the mark; so before handing over your hard earned cash for a new item you would be well advised to at least ask where it came from.
Another way of getting hold of spares is to buy reconditioned, exchanged units. You bring along your old item, hand it over to the spares stockist and in return you get another one which, whilst not new, has been overhauled. This type of product is usually limited to the more expensive parts such as engines, gearboxes, back axles etc and sometimes the products are renovated by the original manufacturer or a company acting on their behalf, sometimes by a completely different business. As ever the old rule of Let the Buyer Beware should be followed and for example let us consider a reconditioned engine; presumably the main bearings have been replaced? Has it been re-bored and fitted with new pistons? If there is a cam belt, has it been changed or merely checked? Have the new parts which have been fitted to it been manufactured to the original specification? As ever, you usually get what you pay for and if the exchange item you are buying is particularly cheap you should make it your business to find out why.
The cheapest way to buy spare parts is to buy from a car breaker. Again, many of these vary considerably in quality and bearing in mind the fact that you may have to fit the item before you find out whether it not it works satisfactorily, and that fitting can cost you a considerable amount of money, you would be well advised to make sure that the car breaker is a reputable person who makes sure that only good quality items are sold. Do bear in mind that for every particular model of car there are usually a number of parts that fail first; there is therefore a pretty good chance that the part that you have had problems with was the one that the previous owner of the car which has been broken up had! If you stick to buying simpler items such as windscreens, light units, wheels etc you should be fairly safe but be very careful indeed if you are buying engines, transmissions systems etc.