You can save a fortune by using spares from car breakers. Almost any car part can be obtained from a car salvage yard and with the internet, you can do the whole business online without even having to go to the yard.
Some parts are obviously more suitable for buying from a car salvage yard than others. Popular items will be body panels and light clusters. If you are buying mechanical parts eg steering mechanism or suspension, you will need to consider whether the part may be already worn out, or may already have been damaged in an accident.
Check out whether there is any kind of guarantee with the used part you are buying. Also make sure that it is the correct part for your make and model. Bear in mind that manufacturers constantly make changes to the spec of their cars. It's worth checking out, if you can return the part if it is not the right one for your car.
Also bear in mind that, whilst you can get parts online, for heavy parts the delivery charge may be too expensive for it to be worthwhile.You may need to search around for older or more obscure car parts for old mg's and triumph spitfires, but more modern and popular cars such as Ford and Vauxhall should be comparitively easy to source used parts for.