Hi Dave,
If you are asking about a gear rather than a toothed pulley, the dimensions (43T, 27.3mm OD) are pretty close to those for a (metric) Mod 0.6 gear.
Go here:
http://www.hpcgears.com/products/spur_gears.htm and at the bottom left select "Metric(MOD)" in the drop-down box. Then, in the same box, select size as "0.6 MOD". That gets you to a list of steel Mod 0.6 gears. Looking at the initial paragraph, you probably want the PG (no boss) style. Scroll down to find a 43T at £9.89 (plus VAT, probably).
They also do them in brass, Tufnol and Delrin (but not aluminium) according to the heading, but you will probably have to give them a call about those.
Hope this helps.
Andy