Hi there. You can find what you're looking for in any neighborhood cotillón (party supply store). I'd just start walking around your neighborhood (they literally are usually located one every few blocks if you live in a remotely shopping related area--I live near Rivadavia and Acoyte and there are literally at least 5 within 10 minute walking distance) and look in galerias, or street front stores. If you have a portero in your building you can ask, or start walking into stores and asking people if you don't find one within about 30 minutes of starting to look. If you are in Palermo I know of a really nice on on Costa Rica near Gurrrachaga. Best of luck and have fun!
If you have time and know how to move in the city, Barrio Once is the place where you will find a great variety of cotillón and good prices.
Ticoral is a well known cotillón store:http://www.ticoexpress.com.ar/