There are many sites out there, the best around is sortitoutsi.net and goal-kick.com is ok if you can't find what you need on sortitout.
Download them one you found what you want, if they come in a file called a .rar, you will have to go to download.com and get a program called winrar, download it and install it.
Go back to the face pack you downloaded, right click it and click extract here.
You'll now have a folder, right click on it, click cut, open my computer, c:drive, program files, sports interactive, football manager 2007, data, graphics, you'll be face with folder, it's easy enough to find your way from there, once you have found either the right folder for the face pack or one close to it, just right click in a space and click paste.
Now to get it to show in the game, open the game up, click on preferences, then unitck the skin cache option, tick the always reload skin on confirm, then click confirm.
Job done, sounds harder than it is but you'll soon get used to it, but it really makes your game look great.
For a faster way check out this thread for another option.
Link but read the whole forum before downloading to make sure.