How do you buy an ignition chip for a 1992 Subaru Legacy?

If you have a turbocharged car, there are several sources, check the Impreza and Legacy owners groups web sites for ideas. If you have a non-turbo (normally aspirated) engine, there is not much you can do that Subaru has not already done for you. Make sure your air filter is fairly clean, muffler is the stock Subaru part (flows a lot more air than many aftermarket replacement units), and your fuel injectors are fairly clean. You already are blessed with approx 9.2 to 1 compression ratio, 4 valves per cylinder, and free-breathing cylinder heads. Just the stock motor generates a load of power for 2.2 liters, and can be revved to 6,000 RPM mercilessly if all maintenance is correctly done. Check for dragging brakes to improve performance a bit, too. The Legacy engine has a fast flame front combustion chamber, and does not use much ignition advance.