You need to be a bit more specific.
You can get leader in acetate and polyester base... the acetate can be
cemented, the polyester cannot.
You can get long and short pitch, the short for camera originals and the
long for prints.
You can get the regular thin-base leader and you can also get super-thick
leader... the super-thick stuff is good for sending out with prints because
it won't shred easily if someone puts them through an autoshredder, but it's
no good for camera originals.
A good source for editing supplies in general is The Boston Connection,
http://www.cutfilm.com. They have most of the supplies that Christie's
carries, and they are generally cheaper than Christie's.
You can also buy some leaders directly from Kodak the same way you buy
film. Last time I needed simple single perf white acetate, though, and
they wouldn't sell me less than 12,000 feet minimum.
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