I'm confused. You want a new subframe, but you don't want to drop the existing subframe?
I'll assume you will have a shop do this, then. ANY shop that would install or remove a rear subframe will have welding capabilities. Why would they NOT if you are a paying customer? Take it to a Pepboys if you must, tell them the tab on your subframe broke, needs welding, and they'll do it. They charge you a rapage, you hand them your credit card, they do the welding (quite possibly while it's still on the car), and you come back an hour later with lube in hand and pick up you ride.
Now, all joking aside, there used to be a complete rear subframe in teh classifieds, an ATI one at that, but it doesn't look to be there anymore. Just to throw my opinion in the ring, it will cost you A LOT less to just reweld the mount on. Shoot, a mom and pop muffler shop would probably do it.
Or call a machine shop, they usually have many tig and mig welders on site that could easily button it up for ya, and I wouldn't imagine they'd charge you very much to do this. You will probably want to remove the sway bar now. Take the broken piece and the car there, and get it done.
