A unique Carbon Bonded Carbon Fiber (CBCF) insulation was developed to provide thermal protection to the isotopic fuel in Radioisotope Power Systems. The microstructure of CBCF is comprised of chopped and carbonized rayon fibers bonded at intersections by carbonized phenolic resin. Production of CBCF insulation at ORNL has been sustained for the past three decades by a single lot of rayon purchased from North American Rayon Corporation (NARC) of Elizabethton, TN in 1987. NARC ceased operations in 1996. Although ORNL has a seven-to-ten-year supply of rayon at the current rate of consumption, an effort has been initiated to identify a new source to meet long-range needs. A summary of findings from the ongoing search for a fiber to replace NARC rayon for production of CBCF is presented.