The online SG catalogue is at
http://www.allworldstamps.com/However, it's not much use for what you want - it used to be possible (but rather laborious) to cut and paste into Excel, but they've redesigned it to make this a waste of time (columns come out all over the place).
If anyone has cracked this, please let us know!
Ther trouble is that (like the other big stamp catalogues) they jealously guard their copyright on catalogue numbers, as they want us to buy their paper catalogues. So they do all they can to make it hard for us.
And I am pretty sure that if someone did put up a website with SG numbers, SG would regard it like a stamp Napster and get it closed down.