Where can i get “special under the hood” parts for my peugeot 406 1.8i 16v LX 2001 model? i.e Turbos etc

Short answer - don't bother.

Long answer - If you want to fit a turbo it will cost thousands as you will need to change the manifolds and therefore the inlet ducting and exhaust front pipe, you will also need to fit an uprated map sensor and a custom ecu otherwise the standard ecu (which thinks the engine is normally aspirated) will see too much air going out the exhaust and will drastically over fuel the engine leading to extremely poor combustion. You will also need to change the pistons and conrods to lower the compression ratio which requires a total engine strip down.

In otherwords it would be much easier and cheaper to fit a turbocharged engine from a different car.

By asking this question you’ve demonstarted a shocking lack of knowledge about engines, so you would obviously be paying a garage to do this work for you. To be honest the cost of fitting a turbo would exceed the cost of buying a different automobile!

The only things that you can fit without upsetting the running of your automobile will be an exhaust and an induction kit, and they MIGHT get you an extra 2 or 3 horsepower at the most (regardless of what the marketing [which is aimed at idiots] tells you). Anything more complicated (if you can find it) like a high lift cam will have a detrimental effect on the engine management system unless it too is retuned at more expense.