I bought a new sprocket and...

And when I screw it on it's too close to the flange and spokes so the chain rubs it. So basically the spacer part that is part of the cog is less than the old sprocket that I had. Is there a spacer, or large washer that I can buy? I'd never thought about different width of the cog before...

sounds odd... stupid answer: sure its on the right way round?

Stupid but potentially relevant though!

Use a spacer from a road 9/10 speed bike cassette then tighten the fuck out of the sprocket AND lockring, that should sort it. I have assumed the sprocket is the right way round.
I have seen it done on a friends bike.

Yeah it's definately on the right way round. I've never seen this, and I've put sprockets on a fair few wheels. I'm thinking of a botch, any ideas in that department too?

Is your chain a Wippermann or Extra fat?

what kind of sprocket is it?

The botch would be to use a bottom bracket space behind the cog, allowing you to add 1-2mm and move the cog away from the hub.

If the cog is a cheapo cog then I'd just buy a new 'quality brand' one. If its a decent cog, then the spacer maybe the only solution.

Sheldon brown has a decent list of shoulder widths for cog, which may be useful in choosing a new one:
http://www.sheldonbrown.com/chainline.html

Looks like a DA or EAI cog would be the best bet or Campy (for ISO threaded hubs).

the cheap cog i had on my condor meant the phat chain was a fag paper away from the spokes changing to an EAI cog brought the chain further away so maybe an EAI cog would help?

+1
but an EAI cog shifts your chainline line out about 2mm...

Jake - I have loads (30-40) spacers in different thicknesses, everything from 0.5mm to 5mm - I have collected them from various bike mechanics over the years - drop in at Condor, Cycle surgey or even Evans and ask for a spacer or two - they will normally not charge.

Failing that, work out what you want and email me.