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.