EDIT: Rahul beat me to it
http://www.vexrobotics.com/versaplanetary.html
Go to this URL and check the same boxes I have in the image.
This should be everything you need, including the BAG motor. If you already have the motor, then uncheck that last box.
To mount it you just use the two free holes in the output block to mount to a plate, tube, etc. (they're 8-32 clearance holes).
If you end up deciding to change what motor you want on the gearbox, the package comes with the mounting plates for the BAG, 775, 550, and AM 90-15. If you ever decide you need to change the ratio, all you need to do is buy the necessary gear kit stages. To buy just those select from only the middle section: "Gear Kit with Ring Gear Options".
Hope this helps.