This side of an arcade I too have not seen anything to better a keyboard and mouse but it seems MS blocked such a method from the 360 and do not look set to change it any time soon.
There are a bunch of devices that translate keyboard and mouse commands into 360 readable commands but it gets a bit tricky.
First give up any real hope of having a PC grade replication but what is out there can get good- some lag exists as the way a mouse works varies a bit from the control sticks but the autoaim can usually make up for it (although I sometimes felt like I was playing an old RPG like might and magic or oblivion).
On the commercial side of things you have the likes of the XFRAG 360 from Xecuter and the XCM XFPS that latter of which you should definitely avoid anything below version 3.0.
Most of the time you are looking at around $100 USD for this sort of thing although if you look around you can push this down to just over half.
Places to get them- you have the usual online shops that sell flash cards, amazon seem to carry a couple and I have seen them in various places in the real world too but with the usual massive markup.
Another twist on that came from some controller modding types that use your PC to do a bunch of grunt work.
http://xim360.com/?page_id=9
I have never tried one but equally I have heard good things vs the adapters.
Also people have done other things like:
http://www.xboxhacker.net/index.php?topic=12347.0
On a more crude front others still have essentially electrically remapped the controller (wire to wire sort of thing) and I have toyed with the idea of something like
http://sensi.org/~svo/%5Bm%5Douse/
