Most anonymous option using most services is cash in the mail or a postal money order. Most other payment options probably required you to ID yourself at some point. Some people are OK with that but ideally you are buying anonymous money completely anonymously.
This is a good reference : https://bitcoiner.guide/nokyconly/
The best is to buy from someone you know, in person.
Trading your products/services for bitcoin is another great way.
From the guide I mentioned above :
"How else can I get some no-KYC bitcoin?Earn itSell unwanted goods for itBuy it from a friend or at a local meetupProvide value to others and have a donations pagePay for dinner when out with friends and ask them to reimburse you via bitcoinMine it"
And then these :"
Hodl Hodl
Bisq
RoboSats
Local Coin Swap
Agora Desk
Peach Bitcoin"