It's precisely governments and anti Bitcoin forces that can label Bitcoin that is seized from money launderers, hackers and other criminal activities as dirty Bitcoin, instead of calling it seized Bitcoin. The fact however remains that Bitcoin is not a dirty cryptocurrency or digital cash, it is decentralized and is hodled and spent by legit individuals and institutions including criminal individuals and organizations. I wonder if seized fiat currencies are called dirty, I seriously doubt it, but it's very convenient for people who wants to give Bitcoin a bad name to call it dirty and tainted and they will still go ahead to sale what they termed as dirty and tainted.
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.
No matter how funny it may seem, I initially learned that supposedly “dirty” Bitcoin was being sold on one of the carder forums before I learned about the real and, in my opinion, the noble purpose of Bitcoin. At the same time, the forum on which I saw such a sale was also accidental for me. At that time, I could not understand that money could be called “dirt.” And I’m probably not alone; if this happened to me, then someone else would also receive incorrect information if they did not subsequently try to find out the details for themselves. Be that as it may, many here claim that there are no dirty Bitcoins, and I may agree, but what can be said about the exchangers that now perform AML checks? In particular, my former signature company, BestChange, recently made it possible to check wallets.
https://www.bestchange.com/report/