How much are you willing to spend?
If you do it yourself, you it will cost way more of your time (and money) than to just buy it.
The problem with a file system is testing it. It sounds easy enough but it is not.
Just because you write a few files and read them on you PC does not mean problems won't
come up later.
Whats more, there are several different kinds of SD cards, that need support and testing.
Even the size of the card matters.
Then you will need to optimize it as running a file system with out sector buffers the size of the the flash sectors (32K) means it is slow, very slow unless you tweak the heck out it.
Trust me, I've done it.
If you are asking if there is a free, well tested, reliable one then the answer is no.The reason why is if you went through the trouble to do this, you would not want to give it away. Plus there is never ending support issues.
You can buy code, but in the end you will find that the dosonchip is a good way to go. No memory used, no code to port, if you change micros no problems.
Here is one you can buy, read the site it is just the first one I stumbled upon, I am sure the are many more.
http://www.blunkmicro.com/fat.htm