I'm not sure why the previous owner would install a PC5 and AT on a stock configuration... that seems like a fantastic waste of money.
On the bright side, if they're already on the bike and you know you want some exhaust changes, it saves you the money! Unless I'm mistaken (which I very well may be) the cat is located in the collector between where the 3 headers come together and where the 2 mufflers split off. You can buy the Triumph Off Road silencers which will give you a nice sound, but they are just slip on so you won't be eliminating the cat. Some folks on here should have experience doing a cat delete on the stock Roadster system but to my knowledge Triumph does not offer a factory solution (for the Roadster... I understand they have something for the Classic that might be modifiable to work).
If you remove the cat and put on the TORs you should consider opening up the intake as well. Removing the stock air system and putting K&N filters directly on the throttle bodies seems to be the most accepted method. This is inexpensive but DOES generate quite a bit of new noise out of the left-hand side of the bike. I love the sound, but it bothers some. There are tons of threads on both the exhaust and air mod topics, and lots of helpful knowledgeable folks here as well.
As for available exhausts, I've found only a handful.
Reband (of Australia) makes a full system for about $1800, Carpenter Racing has their Brute full system for $1200, Zard makes a couple for about $58,439,424,309,487.99 (plus tax), Jardine "makes" a pipe for $1500 but don't expect to see one ship for a year or so.
If I were to make a suggestion, I'd say look at Staintunes. They have exceptional quality and make a nice collector and muffler that fit the stock headers. If you're trying to escape for under $1000 you may be out of luck, though.