Not sure on tilapia, but if there is a fish market you can check in Recoleta (1500 block of Rodriguez Peña, odd number side). If in San Telmo, there is El Delfin (Azara 99), but it's better to call ahead if your spanish is good enough to ask if they have it and understand their potential responses. Alternatively, you can try the Jumbo on Bullrich which we've had references for hard to find fish, but I cannot attest to the quality/freshness there.
[background=rgb(252, 252, 252)]Time your fish purchase with the opening hours of the central market. It is closed on Sunday so Mondays barrio chino fish is leftovers also in most pescaderias. Tuesday is delivery day and everyone goes. If you are looking for popular stuff go early. I think Thurs Fri is also good but not as good as Tues as Tues . [/background]