Hi, I'm assuming you have the stock low mount exhausts. Take a look at this tutorial for chopping them and making them a little louder.
http://ducatimonsterforum.org/index.php?topic=2312.0If you can muster up the tools, it doesn't seem so bad. I think straight pipes are obnoxiously loud but other board members would know better if that's harmful for the bike or not. As for aftermarket brands, look up Danmoto, Leo Vince, Marving, Sil Moto, etc. Many of the sponsors on this website carry aftermarket pipes for the older monsters!
It is a very simple mod...and is well worth time and effort!!
On my 98' M900 I literally cut my two cans down to 11'' reinstalled the cap, and couldn't be happier with sound!!