I see, good to know that not all embassies offer such documents. For me, it was easy to obtain the proof of income from my embassy and it took less than 15 minutes. Submitted to the immigration with no issue.
Another option you have apart from 400k in the bank account, is that you get a proof of income document from your embassy and submit with the rest of paperwork to the immigration for the Non-O (Thai wife).
Just to be clear, it's has to be income from oversea, and it have to be at least 40k baht or more per month.