Where do I find a good ADSL surge protector?
First, ADSL is a radio wave. Any high capacitance protector will degrade DSL 'radio' signals (for the same reason that conventional telephones eat DSL and need that blocking filter.) A DSL protector must be low capacitance to not 'eat' radio signals.
Second, the protector is not protection. Protectors do not stop or absorb surges. Effective protectors connect surge energy to the protection - a separate device. Protection is earth ground. To provide protection, a protector must connect short (ie 'less than 3 meter) from each phone wire to earth. This has been routine in every telco CO everywhere in the world for the past 100 years. To make a protector even better, a lower capacitance protector is connected even shorter to earth. Every meter shorter increases protection.
Third, distance between a protector and ADSL hardware increases protection. Wire has low resistance but higher impedance. Wire impedance is why wire length is relevant. An even shorter distance to earth means a better connection from telephone wire to earth ground - lower impedance (not to be confused with characteristic impedance). Increased separation between the DSL modem and protector means higher impedance. More surge harmlessly absorbed in earth when a protector is connected shorter from phone wire to earth. Less surge will find earth ground destructively via your ADSL hardware.
Important is how that wire connects to earth. For example, sharp wire bends, wire bundled with non grounding wires, inside metallic conduit, using a different earthing electrode from AC electric and cable, wire splices ... all adversely increase impedance to ground - decrease protection.
Better DSL protectors use semiconductor devices such as Sidactor or low capacitance avalanche diodes (such as
http://www.semtech.com/pc/downloadDocument.do?id=532). At the consumer level, the protector must specifically state in numeric specs that it is rated for DSL operation. And, of course, it must have a dedicated connection to earth. Protectors do not stop or absorb surges. Protectors simply divert surge energy harmlessly in earth - and remain functional even after a direct lightning strike. You will be amazed little a protector need be when it diverts (does not absorb) energy even from direct lighting strikes.
Meanwhile it must also be low capacitance so that it appears non-existent to ADSL signals (radio waves).