There is a place on Cabildo between La Pampa y Lacroze (sorry I can't be more specific, I can't rmember exactly where it was, but it was on the north side of the street) -- the sold a number of foreign lines, including Laura Mercier so I suspect they may have Chanel.
A warning -- the prices for foreign cosmetics are much higher in Argentina. MAC -- which in Canada is essentially an entry level company -- offers powders at home for $20 dollars. Here the same item is $45 dollars. Products from foreign companies are regularly featured in Para Ti magazine with list prices of well over 150 pesos for items that are only $20-30 dollars in North America.
I used to buy Bobbi Brown at home, and still do when I get the chance, but here I have switched to buying Avon -- they have very good quality products at reasonable prices -- I think their press powder was 35 pesos, has a nice case and good sized mirror. The packaging is very nice as well (which is really what you're paying for when you buy high end cosmetics anyway). They have a number of Avon beauty Centres around town where you can purchase all products -- there is one on Sta Fe near Callao I believe.