Sprite sheets are a collection of single frames. Once you have 2 or more frames, you can animate your mate selecting another image every 20-100ms.
Download sprite sheets
On the web there are some resources where you can download some sprite sheets:
spritedatabase.net is a database with a lot of images from games. They are able to rip the sprites and create sprite sheets of the game characters. Unfortunately you don’t have any preview, so you have to download every time the zip to see if there is a sprite sheet inside.
www.spriters-resource.com is another database with a lot of images from games. Since it is not really for sprite sheets but more for sprites, it is not easy to find the right resource.
www.deviantart.com is a collection of images from artists. Not for spritesheets, but you can find some. Maybe someone will create one for this application one day :P
Create sprite sheets
It is also possible to create a sprite sheet from single images. HERE you can find some utility to download.
And some other tools: