Brett if you are trying to buy ready made worm and gear you could try a bearing supplier. I checked into it some time back and was somewhat dismayed by the pricing (from memory it was looking like about the $400 for a set - one worm and one gear). I did not do a lot of investigation so I don't know just how good they are for our purposes.
Hobby Parts at Mansfield (they do mail order) also supply some.
http://www.hobbyparts.com.au/ Again I don't know how suitable the gears are for driving a telescope. I have purchased via the web and the delivery turned up the next day which was impressive.
Bob