On my road bike, I have full Dura-Ace 7800 groupset (10-speed), but I have just been informed that the front chainrings may be in need of replacement.

What am I limited to in terms of replacement? Should I just bite the bullet and buy a straight Dura-Ace replacement with the same ratios? Or can I be flexible?

Finally, is it worth looking at Ebay for a used (but better conditioned) replacement?

7800 you should be able to fit any normal 130mm 5-arm chainrings to. You might have to file the tabs off the chain rings as shimano have a slightly different shape to others, but the bolts line up in the right place.

The DA ones are horrifically expensive, but with the shaped cranks anything else looks a bit rubbish. Swings / roundabouts.

SJS are a good place to start.
http://www.sjscycles.co.uk/mobile/stronglight-dural-130mm-bcd-5-arm-outer-chainring-prod18497/