What variety do you want?
Online (on mercadolibre) you can find several people that will sell you seeds for different varieties, with prices ranging from AR$5 to AR$ 20. Some supermarkets might also have them (yesterday i saw onion seeds at Jumbo, and Easy also has seeds).
The INTA has a program called ProHuertas. You can also get seeds from them (don't go directly to the INTA, but rather to the Ministerio de Desarrollo Social).
If you get your hands on any non-supermarket or verduleria tomato (as they are most likely hybrids) you can plant the seeds. If you plant the seeds of hybrids your tomatoes could look nothing at all like the one you took them from.
Again, depending on the variety you want, I could give you some for a local variety called "tomate platense". You can see pictures of this variety here:
http://tomateplatense.wordpress.com/fotos/. If interested and are anywhere near downtown send me a pm.
For plants (probably the best option at this time of the year) you should be able to get them at almost any plant store.
Guide for germinating tomato seeds:
http://guia.mercadolibre.com.ar/germinar-semillas-tomates-y-otras-variedades-55440-VGP (in Spanish)
Guide for growing tomatoes:
http://gardening.about.com/od/growingtips/tp/Tomato_Tips.htm