Hey mate, think i could help you a lil bit.
if youre looking for gear, altough as the fellow above said, its quite more expensive (not that much if you have dollars), you should check Talcahuano Street around at 200/300, where meets Sarmiento Street, over those few blocks you have at least 20 music stores. Its in the center of the city, if you need adresses just ask.
about the jazz clubs, there are quite a few, let me know what region of the city are you staying or whatever and i'll tell you whats around.
