8 1/2 independencia, there is a music shop opposite KFC (on the right) before PizzaHut on the left heading out of the city. You'll get a really crappy guitar, maybe!, but you won't find a keyboard for $100. There is also a music store on the second floor in Plaza Central., I've seen ok guitars in there for 7,000RD$, but still bottom of the rack in quality and double what you want to spend.
Are you beginning or already play? A $100 guitar will be really bad quality, and sound terrible.
Consider second hand, pop to one of the university canteens and have a look on the notice board for sales.