Does anyone know of good travel agent who can help me book my internal flights in Argentina?

You're most likely going to be flying on Aerolineas Argentinas/Austral, so you might as well book your flights on their website rather than go through a travel agent. You don't even need paper tickets, just your passport and a print out of your confirmation page when you check-in.

http://www.aerolineas.com.ar/ar/index_en.asp?pais=us&idi=en