I don't know if there is a muffler only application for the Focus, but if you look at your muffler, its kind of an odd shape and i don't think you could find something that would a) fit the space, b) be "high flow" or c) worth the effort to install. Part of the reason to replace your stock exhaust with a good cat-back system is the restrictive nature of the entire stock exhaust system-It's not just the muffler that holds you back but the entire system. If you are interested in saving money, I would strongly recommend saving up and getting a good cat back system. I have the MBRP cat back, I really like it and recommend it but there are other systems out there that are at least as good. If you do install an aftermarket exhaust system, be ready for increased noise that you get with any cat back. Some are louder than others, some people like the sound, some like "quiet" systems.
There are a lot of threads here about exhaust systems-do some research and check out peoples opinions as to what is the best system but whatever you do, don't compromise on mods to yor car-take the time to decide what is the best route to go then do it. There is a lot of knowledge out there.