I’ve always relied on TechRestore for cheap upgrades or repairs on any Apple products I’ve owned, but if you prefer DIY repairs, try the
new iFixit website, which will sell you the parts and then provide detailed guides on what to do with them.
Remember, most manufacturers (including Apple) don’t want customers doing this sort of stuff. It will void your warranty, curdle your milk, and make your fingernails fall off. However, if you’re out of warranty and don’t have an extended plan like AppleCare, it may be your best option.