I think this is not too complicated
You can purchase BTC through any source (exchange, p2p, trade) and then exchange the BTC on any anonymous exchange platform like Majestic Bank.
You can find their Onion here:
https://majestictfvnfjgo5hqvmuzynak4kjl5tjs3j5zdabawe6n2aaebldad.onion/
I do not give any guarantee for the safety of links.
Anyway, there you can exchange BTC for XMR, it is very convenient. There are houndreds of websites like this in the web and darkweb.
To purchase BTC 100% anonymous I could recommend anonymous bank accounts, they are simply opened on the name of other people with the use of fake IDs..this can be tricky with bigger banks but I can help with that for many different countries.