Fileshack but its elsewhere as well, and probably torrents.
Trackmania is an awesome arcade style racing series. One of the big draws is its very easy to use track editor. With custom tracks coming in at only a few kb each, when racing online you can play custom track after custom track. Very fun.
Theres 2 versions out now. Trackmania United Forever is the retail game, comes with all the old environments from the older TM games. All the different environments feature different cars with different handling. So playing the Rally environment is different from the Desert.
Trackmania Nations Forever is the free one. It only comes with one environment, Stadium. Other than that you get the same track editor and online play. In fact with the Forever update, Nations players can play with United players on the same server (as long as the server is running Stadium tracks).
So go get it. It be fun, yo. And for those worried, this update removes all starforce!