Signal amplifiers are availabe at hardware stores and home centers.
They will usually carry the Phillips, or RCA brand. Both are good, and provide about 10 db of gain, they also make a 20db version. You may have to experiment with where it should be placed in the circuit, but generally it will go at the beginning of a long cable run. These are boadband amplifiers that will boost tv and cable signals. If you have cable tv and use the internet on that same cable, you will need a more specific type called a "bi-directional" amp.
This type boosts the signal coming in and going out, so you can reach the internet.

Hope that helps,
Mike - TurningColorz

Our cable company put one on for us - you might want to call them and tell them about your problem and they might just put one on for you. We use Cox.