Car parts question

Guys, where do you buy stuff for the car?
any car markets in Warsaw or something like this, sometimes there is a need of stupid little plastic pin, and I don't want to go to a mechanic for it, coz i can fix it myself, but i just don't have that plastic, so i will have to go to a mechanic.
Actually very good car mechanic if anyone in need, I can recommend him :)

What you are looking for is available from your car maker dealer.
You can also buy generic car parts, they are way cheaper, and sometimes the quality is very close to the original, just because they are made by the same factory but not branded. In polish they are called "zamieniki ".