I personally like dealing with Crutchfield. They tend to be more expensive, but their support is (in my experience) unequaled. They have a good tutorial about choosing an amp and installation steps needed. You might want to use them to compare and narrow down your choices. After that, if you need something more quickly or at a better price, you might want to check out your local car audio stores. For online purchase, Crutchfield is great, but there are a lot of other places out there as well.