@Guilhem *****
, okay, lets go over this one more time :P
You did NOT enter thailand on a tourist visa that you bought from a thai consulate before you came here.
You entered on a 30 day visa exempt entry
AND
there are plenty of appointments for people who entered on a 30 day visa exempt entry stamp to go and get the 30 day extension with the embassy letter.
SO
get your embassy letter, make an appointment for the choice I showed you in that image I posted and go get your 30 day embassy request extension :)