We used two worms for our arms last gear with great success.
Biggest drawback to a worm is efficiency - about 65% at best - so you lose a lot of your available power. I would look at using say the Dewalts.
With that being said I found some nice worms at
http://www.qtcgears.com/
You might also search ebay.
__________________
Ken York
Mentor/Coach Team 476 Ponca City OK
An ignorant person is one who doesn't know what you have just found out.
Will Rogers