Here's an excellent source for parts, in Australia.
Example of worm gears available:
http://www.smallparts.com.au/cgi-bin/store/datafill.pl?...c-Metric&Step=3=post
Note that metal gears have a minimum production required, so you have to e-mail for pricing information.
The main index:
http://www.smallparts.com.au/cgi-bin/store/product.pl?alpha=ALL=post
If you can't find it here, you don't know what it's called.