National Trading Online provides "Total Class Issue" which is the number of shares on issue for a particular stock, i believe that market cap is just the Total Class Issue x Close price, i haven't worked out how to get that info from ComSec yet though
It should be quite simple to find out the market cap of any company.
If your using an online broker just use their software to find out the market price, it is usually provided somewhere. You can use the ASX site itself like another member just suggested.