Texture King has a large database of textures. The website is easy to navigate and downloading is super simple.
Image Abstraction also has a great database of textures and is well organized. I found downloading to be a bit easier on Texture King, but it is still easy to download at Image Abstraction.
Texture Spot's database is not as large, but still worth taking a look.
This is my favorite place to find textures from my favorite web company. Google image searches always helps you get what you are looking for.
Go a Google search for the texture you want, click the Images link above the search results and when the results come up, press the "Large" link for image size to make sure you are getting the best possible textures.
Whenever you download textures, make sure you read the terms and conditions from whatever website you are downloading them from.