Anyone know where any bulletin boards in the Centro area are, where I might post teaching offers? I found one in a papeleria and another in a bar but so far that's it. Seems like they are all in the 'burbs unless I'm just missing them.

Usually, you have to just walk around and introduce yourself to people in different shops and ask them if you can leave some business cards or a little poster. Try the few photocopy shops in the area. Although, if they don't have a bulletin board (Tablón de anuncios), somebody will likely soon take them down if it wasn't the boss you were speaking to.