I'm also curious at why the OP will ask where to buy dirty Bitcoin, like you I believe that the best place to make such enquiry is in the dark web. A reputable forum like bitcointalk is not the right place to get information about illegal activities.
If it really exists and the price is lower than normal, P2P wallet transfer is supported, and can exchange it for cash in my country
Op answered but I still don't understand
they want to buy dirty bitcoin which they think are sold at a cheaper price just to exchange back to their fiat money? it doesn't make sense if we follow op's logic.
if by op's logic, dirty bitcoin is sold cheaper than those not previously linked to illicit acts then you are also not going to make any profit because you are also going to have to sell at a cheaper price, so I do not understand the thought process behind this question