If the only access you have to car parts is through your mechanic, you may want to consider ordering the parts through an online parts exchange. These e-commerce websites allow you to find and source your own parts, which should go a long ways towards reducing your costs.
From this point, you will need to find a mechanic prepared to install the parts you want; some of them may want to provide the parts themselves, so that they can avoid any potential liability, as well as make more money on the transaction. The majority of mechanics will be prepared to do so however, although if you want to reduce your costs even further, then you could try and install it yourself, and then take it to a mechanic for a check-up to ensure everything was done properly.