The elbow? Unless you can get one from a car that's being broken, they're pretty-much a custom job.
As for 2.5" front pipes; HKS/Blueflame/Mongoose all make one that's a direct replacement from the elbow to the cat/decat.
I think 2.5" is just about right for 300bhp; much larger and you'll start attracting attention from the noise. At 2.5" it still looks fairly standard too. - I'll sit back and wait to cop an earful of flames about that... but if you're not looking for 400bhp I don't think you need to go to 3", and it's only getting harder to run a free-flowing exhaust.