Tigre is a good place to look for a moses basket and mattress, but they mostly come with stands as well. When we were looking there were a couple of shops on the streets just outside the main market that had just baskets.
You could try a baby shop like blue on Plaza Italia, or there are a couple of shops on Santa Fe at about Puerreydon if you want a carry cot ... but I'm not sure if you would want to use this all the time as the mattresses are usually quite thin.
If you do find a basket nut no mattress there is a mattress shop on Las Heras and Coronel Diaz where we bought our cot mattress
HTH