Big thumbs up to the guys at SKP (meant to post something up a few days ago) - and if you want those green arches in the pic on their website, they've gone! 8)
I asked the same question a few months ago.
Bought a few 'new' repro parts (such as step, inner outer etc) and the finish and fit was enough to persuade me to seek some orriginal cut-out arches.
Emailed a few places - SKP replied within a few hours, with price etc. Then very kindly sent me loads of photos, waited for me to decide what I wanted to do, and were more than happy to let me go and pick them up and have a look at how they would fit them...I had to cancel the pick up last minute, so they couriered them to me for a decent price, the day before they were jetting off for a few weeks - I think they may still be away, or perhaps just got back (maybe with a load of 'fresh' parts.
Excellent customer service.
I am currently fitting the arches - I'll post some pics if I dare - this is a new one on me, slightly ambitious (the arch fitting, not posting pictures)
Had another quote from a company - quick reply, but £150 for the arch - no problem, but add on the £30 postage and VAT and it became much much more expensive.
The other big company I emailed replied about 2 weeks later - could have been away I guess!
Anyway - SKP are the first people I speak to when I need some more bits....good chaps IME.