On arrival by air you can pickup your tourist visa for $20USD or business visa for $25USD, If you are travelling by land though you will be charged more by the border guards. They will ask for anything from 1000 THB to 1200THB for a tourist visa. The correct charge is $20USD. Tell them that if they don't accept it you will complain to the Ministry of Tourism
Make a copy of these two cards so they will know that you mean business. Also if you do stand up against this corruption make sure you check your passport and the stamps or lack of as they may not stamp you in, which will cause you trouble later.
If you want to remain in the country longer than 2 months it's worth getting a business visa on arrival as if you change your mind later you will need to leave the country and re-enter. A business visa is $25 for the first month.