Try
http://www.zipperstop.com/ . I bought some separating zippers from them off Amazon and was able to get exactly the length I wanted for creating zippered leaders for my longarm frame. Aside from their website and Amazon, they also sell on eBay.
Edit: I will say that the choices on their website are overwhelming. I'm glad I didn't have to wade through all of those options to find the zippers for my quilting leaders!