To be clear, this is not my preferred method. I was very distressed this weekend, and now that I can think more clearly, I have abandoned this idea. It might be a method for urgent situations, especially for people who already know how to get methadone (or other opioids), but it carries a lot of risks. I do not recommend this. The problems:
1. You need A LOT of the stuff. Where I live, addicts get methadone as a liquid solution containing 10mg/ml. The average adult needs at least 1g to ctb. More if they're heavy or already used to opioids. So somebody trying to ctb with methadone would need at least 100ml of the methadone liquid. Seeing as the starting dose for the average addict is 20mg a day, you'd need 50 doses of the stuff. Good luck getting 50+ addicts to sell you their daily dose... (or one addict to quit cold-turkey for 50 days) That would be fun, at that point you'd have cured somebody's opioid addiction. Jokes aside, that seems unrealistic.
2. Risk of failure is high, as with any type of OD. And the consequences of failure are really bad: Failure in this case means giving yourself an opioid addiction. At that point, you'd be alive and ironically, ready to sign up for a methadone prescription... Opioid withdrawal is no joke.
3. Risks of aquiring drugs (of any kind) on the black market: The usual risks apply, such as being sold something else than what you want, being scammed etc. There is also the risk of getting an unwelcome reputation with the local addict population, if you try sourcing methadone in person. Get on their bad side, and you risk getting hurt. Get too close with them and you're in for bad relationships. Or worse, they might like you so much that they try to stop you from ctb. There is also the risk of getting caught by the police whilst trying to source methadone in your local area, or, if buying online, having the package intercepted by law enforcement.