I look at the two you mentioned, as well as Clarin, argenprop.com, even MercadoLibre, and a few others that I get when I do a Google search for rentals.
Another thing that I have done in the past, and will be doing soon, is get out and drive around neighborhoods that I've seen decent rentals on one of the websites and I may be interested in the area, and look at both displayed rental signs and stop at the neighborhood inmobiliarias. Of course, that usually means renting a car or driver for many people - I own a car, which doesn't seem to be the norm for people living in the city,(which is understandable and I bought mine years ago when I moved out to the suburbs and just never got rid of it - about which I am often thankful every time I need a car

) Going out to the neighborhood by bus or taxi and walking around is an option, but obviously less efficient in terms of ground covered.
People use Craig's List to find temporaries and room/shared space deals, but I suppose there might be a gem or two in there for long term leases (I'm assuming you're looking for long term giving the sites you mentioned).
If you have to pay a commission, I believe there is now a law (I've read it was coming and someone recently told me it had passed - I have been quite busy and missing the details of things recently) limiting the commissions to real estate companies to 1 month. I was stuck paying a 2 month commission more than two years ago on my current apartment or would have had to keep looking.
I'm starting a three-month-long search (hopefully no longer) for a new apartment, myself. I hate looking for long term leases here. I so miss the simplicity and certainty of being able to go to a real estate agency who has access to a MLS, laws that allow people to move into a rented residence with only security deposits and no need to secure the contract with an actual piece of property, being able to sign a 6 month lease if both parties are in agreement, etc. Ah well...