There's a guy in San Telmo on Defensa street. He's there on the Sunday Flea Market. Actually there are several of them.
Just go to the Flea Market on Sunday.
Several years ago, we had one made by http://www.martinianoarce.com/, but his prices have undoubtedly risen since the crisis. Still, if you want to cover your custom casket with fileteado, he's the guy: http://www.martinianoarce.com/ataud.html
I ordered three of them to bring as presents to friends: 150 pesos each, personalized. It took him one week to make it.
There're several guys on Sunday on the Feria de San Telmo, prices are very different. There's a guy on the plaza Dorrego, at the backside and I like his work, he's the one of 150 pesos. The others were asking 200 or 250 pesos.