ciao, I think a good resource for your problem is on
hppt://www.pupman.com
if you join the mailing list a very large number of good guys discuss about Tesla Coils every day... One of the recurrent topics is how to rebuild transformers. Especially NST. The website has searchable archives also. So you can decide if you are interested to join.
Hope this helps a bit.
Ciao Daniele
Created on Thursday, March 15, 2001 7:26 AM EDT by Daniele DB Bortoluzzi