Cellular repeaters are pretty straightforward devices. At the risk of over simplifying, it's a three part solution:
1) external antenna mounted in an area with a good strong signal
2) a signal amplifier to boost the signal within a specific range of frequencies
3) one or more internal antennae to rebroadcast the amplified signal
I've had some experience with the PowerMax line of products and been quite pleased; they are relatively simple to setup and operate:
http://www.digitalantenna.com/
Wilson Electronics is also a well established name for this type of product:
http://www.wilsonelectronics.com/