You tend to find mobile signal boosters are more business type orientated devices. We have one in the back of our store hooked up so all networks get a 100% 3G signal in the shopping center. For the consumer it will most likely be an expensive piece of kit.
I will take a look about and see what I can find for you though a little later this aft
Found this but it is pricey, you put your network details in the bottom of the screen and it recommends which one to go for. If you are on 3 you will need an even more expensive one as their network is 3G only, all other networks though still have the dual band GSM signals :
http://mobilerepeater.co.uk/shop/home.php
You can get these little card things that sit under your battery for considerably less though. Most sites they are getting mixed bag of comments though, might be worth checking one of those things out before forking out a few hundred.
Link for the internal under the battery thingy!!! (Very technical I know considering I work for Voda!!! ) :
http://www.ciao.co.uk/Mobile_phone_I...oster__5330526