It's hard to kill a CAT. Is it plugged up or rattle when you hit it?

Unless you live in California, you can find multiple sites for aftermarket. Find a muffler place to install it, just remember that it will not last that long. The Volvo exhaust system is stainless steel, most aftermarket are not.

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Keeping it running is better than buying new

Look here:

http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.X2004+volvo+v70+cat+converter.TRS0&_nkw=2004+volvo+v70+cat+converter&_sacat=0

I called a few local places here, north of Boston. They are hesitant to quote
a price without seeing the muffler, which makes sense. One good place
here "Lou's custom auto exhaust" a local chain said if it's just two ends
and everything is the right size they start at $80 labor to weld in the Cat.
(Not if the pipes are shot though)

Review the entire system while you're at it.
Here's exhaust system parts options:

http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.X2004+volvo+v70+exhaust+system.TRS0&_nkw=2004+volvo+v70+exhaust+system&_sacat=0

As Herb says, aftermarket exhaust is not the same as OEM.
Mileage varies, I've got 5 years and counting on a Stella system in my 850.
I'm in salt city here, but low mileage.

Bill

Around here you can get a Magnaflow cat welded in for $250 including parts and labor. I had this done on my wife's Toyota that had a persistent PO420 code with good results.

Aftermarket exhaust parts don't last-- about 3 years or so. If the rest of your exhaust system is in decent shape, you can just have the cat welded in at a good independent exhaust shop.

What condition is the rest of the exhaust in? Another alternative is to replace everything behind the cat with a genuine Volvo cat-back system, assuming you are planning on keeping the car for more than a year or two. This option would be way less than $1500 even including an aftermarket cat-- you can price the exhaust at online dealers like Tasca.