I think they stopped making it a long time back. We used to use it a long time ago for a C. nubeculosus line in Aberdeen, but not sure it actually made that much difference and the ones here in Pirbright go fine on parafilm if properly stretched (although they are probably heavily adapted to that feeding method). The other thing I remember was it was a pain in splitting constantly, which may not be ideal if feeding pathogens... Which species are you trying?
We use parafilm to feed our Culicoides sonorensis colonies. Our colonies have been fed using this method for decades so they are probably heavily adapted as Simon mentioned. Other things you can try are silicone, latex, or chick skins. I included some citations below that used these methods. Another method you can try is edible collagen. We have had good success using this with our mosquitoes, but the colony midges do not seem to care for it.
L Davis, E & F Butler, J & H Roberts, R & F Reinert, J & L Kline, D. (1983). Laboratory Blood Feeding of Culicoides Mississippiensis (Diptera: Ceratopogonidae) through a Reinforced Silicone Membrane. Journal of medical entomology. 20. 177-82. 10.1093/jmedent/20.2.177.
VENTER, G. J., HILL, ELAINE., PAJOR, I. T. P. & NEVILL, E. M., 1991. The use of a membrane feeding technique to determine the infection rate of Culicoides imicola (Diptera, Ceratopogonidae) for 2 bluetongue virus serotypes in South Africa. Onderstepoort Journal of Veterinary Research, 58,5-9 (1991).
I looked for NESCO Film some time back and was also unable to locate a source. Try using chick skin or cleaned natural lambskin condom material for initial feeding of wild caught Culicoides. Once the colonized midges have adapted well to the feeding system, you may be able to switch to parafilm and select for midges that will feed through this material.