(A fictional story I made up for entertainment purposes only -- I would never do this nor recommend it is real life)
BTC is your best bet to be anon.
Buy it in one wallet (your real account, device, and IP) then transfer it to another anonymous wallet you only log into using an IP and device that is not directly associated with you (I recommended something dedicated to just this purpose -- and, actually using yet a third wallet between these steps with capability of "mixing" coin) and in addition use a proxy to access it via another country (not in your real name and using an email you have NEVER used for anything other than the wallet). Never open the anon wallet or email using your own IP (And as stated, I would add the proxy in addition to open it from another country's IP). Make the purchase from the anon conditions using the anon wallet.
You would have to be a pretty big fish for LE to spend the money and time to track all that and build a case. Even then, since the purchase was not made by "you" (not your IP, device, wallet, account, email, or Wi-Fi etc), your lawyer should at least be able to present some plausible deniability.
Remember the most powerful tool for you vs legal trouble -- do NOT talk at all in any regard to anything to anyone; simply ask to speak with your lawyer; memorize the phrase "I have no statement at this time and request consultation with my lawyer" -- say NOTHING else.