You can find these at Radio Shack (get a good quality one), and Circuit City has begun selling them too (at a high price).
The best place to put these is at the junction, BEFORE the first splitter. Putting an amplifier behind your TV or before your cable modem will NOT help at all, because at that point the signal is already bad and weak. You will just end up amplifying a BAD SIGNAL.
There are several varieties of Amps, but you'll probably be okay with a one port. These usually have a gain of 12 to 15db. Make sure you get a 900MHz (or better) amp. These will run you from $30 to $120.
Two port amps usually have a gain of 9 to 13 db.
Four port amps usually have a gain of 6 to 10 db.
NOTE: Amplifiers are to boost EXISTING GOOD SIGNAL in order to increase the distance that it will travel. Do NOT use an Amp to fix a noisy or bad signal; You'll just end up amplifying a bad signal.
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