Most local fabricators will have the odd off-cut lying around that can be used, mine cost about 40 quid, just took in the cat to let them see the diameter of pipe required....
If that one has gone try Zorstec. http://www.zorstecexhaustsolutions.co.uk/
fairly local and quality work. Made this extension to clear panniers for me couple of years ago.
Do a quick ebay search for 'exhaust bend' or 'mandrel exhaust bend' and you'll find hundreds.
Just make sure you know the OD of the stub off the previous section (Or ID if you're going to ram it up there) and it'll cost you about 15 quid.....you can get weld on or slip on ones, the weld ons are cheaper (but don't have the slash to tighten with a clamp) and you can take an angle grinder to an inwardly facing part of the tube to make a slash so you can clamp it on if necessary.
I've fitted one before now though that was such a tight interference fit, I didn't have to...I just whacked it home with a soft mallet then drilled and pop rivetted through just in case differential heat expansion made it loose
Pretty much any skip 'out the back' of any exhaust fitters will also have a good number of bits of pipe you could modify with a grinder and a vice