What additional documents should I submit to prove the source of my funds for a visa application in Thailand?

If you are applying for soft power also be prepared to show how you will fund yourself for the duration of the visa

Get a sponsor letter from your parents

Yea they are super anoying. They clearly stated that they want a bankstatement, which i provided. Then they came back and said i had to provide a bank statement….. to which i answered that i already provided it. From this group i know that they are actually looking for minimum of three months of bankstatements. I could provide them this from my savings account without my daily transactions, luckily. I believe it is a great breach of privacy if they see all your transactions. I hold more that 50k euro cash and was able to show them like 37k eur consistently. I told them to refund the money if they cannot make up their minds.

Give proof / receipts to show where your money came from. They don't want money laundering or illegal money and that you have an ongoing source of income to support yourself

Payslips is the most straightforward way to proof it. I submitted statements from kbank and also Payslips. Then added currency conversion (because payslips are in euro) and marked the transactions on the statements. This was accepted.