If you are looking to replace parts of your lawn mower, your first stop would be the manufacturer's website to see if they sell official parts or have any official suppliers. For example, if you have a Husqvarna lawn mower you can visit their site and use the store locator to search for a location near you that does servicing.
You can go online and visit specialist lawn mower sites like Lawn Mower Parts World or MTD to search for suitable parts. Your local Home Depot might sell what you are looking for, and of course there is always Amazon which sells a whole range of lawn mower parts.