Easiest for me is to just go to the desired telephone company office and buy it there. 7-11 stores have difficulty filling out the form that registers the SIM with the government (usually done with a Thai ID card) but must be filled it manually when copying information from the passport.
It is OR. After you get to Thailand, go to the nearest immigration office and ask what THEY consider proof of income in your case. (Seems to differ by office)