Nitrate sticks to the fabric better than butyrate. So it is used as an adhesive and sealer. BTW, you can paint butyrate over notrate, but not the other way around.

Sig probably still sells their nitrate. I have a couple of quarts. Brodak sells nitrate and butyrate that are blended by Randolph for model use. They are a little spendy, but excellent. www.brodak.com
The web site is a little clutzy but the service is superb. Phone and web orders sometimes go out as quick as a couple of hours. If you need more, try Aircraft Spruce and Speciality for quantitiy buys of Randolph. http://www.aircraft-spruce.com/