I see tons of topics about this, so it's about time we just had an instructional one. Mods, please sticky this if you deem it appropriate.  Hope this helps!

1) Go to images.google.com or another photo search engine. This is my favorite.

2) Search the name of the celebrity you want. If you have a particular movie in mind, search that with it too, and maybe add the name of their character (i.e. "Christian Bale Dark Knight Bruce Wayne," "Johnny Depp Pirates Jack Sparrow," etc.)

3) Set the search to limit results to a specific size. If you are looking to print a 4x6 or 5x7 photo, the medium or large settings should do. If you are looking to print a standard-size promo picture (8x10 or 8.5x11) go for the highest setting for the best results. In any case, the larger the picture the clearer the result. Use the biggest photo you can find and you'll almost always get the best results.

4) Print. While you can use standard printer paper, you've gone through the effort to find a photo so you might as well make it look like a real photo by printing on photo paper. You can get photo paper of most sizes at a standard office supply store. Make sure it's compatible with your printer. Many printers make photo paper specifically to work with their technology (i.e. I have an HP printer and use HP photopaper). While make and model of your printer and paper won't have a huge effect, it never hurts to be safe.

5) Send away. Good luck!