You can still get the Byers gears. Check ebay for adverts.
Then you can also get them from here
http://www.aeroquest-machining.com/page11.html .
But the worm and wheel is not going to give you accuracy in goto. It will only give you very good PEC.
If you want accuracy, then look at high ratio gear boxes, and high count optical encoders,. Of course, your mount has to be made right (All perpendicular) and of course good alignment.
Theo