I am building a robot, and I need a gear set that serves a similar function as a rear differential. My goal is to turn an input shaft with an electric motor, and turn 2 shafts in opposite directions from each other. I imagine it as a drive-shaft spinning and causing one rear wheel to spin forward and the other to spin backward. The gears have to fit in a area about 1 1/4 cubic inch of space. Is there anywhere I could get such a gear box, or even the gears and housing separately? My Grade depends on finding this part, any help would be greatly appreciated. Thanx

A gear box from just about any RC Vehicle will fit your needs.
Check out any 1/8th scale vehicle Like an associated RC8
Another smaller size but similiar is the Jammin CRT.5

he needs one that will turn the output shafts in opposite directions not the same. and in one going forward and the other going backwards.

the concept wouldn't be too hard to build a gearbox for. i would start out with something like the old rc10 6 gear trans. leave the 3 gears on one side of the trans as is. remove the idler (center) gear on the other side and figure out a way to move it and add a forth gear in between the top input shaft gear and the bottom output gear. the additional gear on the one side will reverse the rotation of the output shaft in the manner you are needing. it will take a little fab work but shouldn't be too hard to build. now durability depends on how well you can fabricate.

I believe that Tamiya makes gearbox sets that you can buy. I did a quick search on the TamiyaUSA site. Here is a link that might be useful
http://www.tamiyausa.com/product/category.php?sub-id=38000|39000|42900|37000

The other thing worth looking into is Erector sets or possibly Lego. I believe both have some sort of gearboxes that you can build/buy. Here is an interesting link describing some things that can be done with LEGO gears http://myweb.cwpost.liu.edu/magot/classes/csc/RobotFundamentals/downloads/gears_drivetrain.htm

This has alot of small scale robot info,

http://www.servomagazine.com/

Speaking of legos, i know for a fact you could build a pretty small one with legos. Get some technics and use the bevel gears. One shaft on one side and two on the other. Three gears. The two would spin oposite of each other. Now they are plastic, so wether it will work will depend on how strong they need to be.