Octomyxa has been a problem for those of us trying to karyotype plasmodiophorids. The late Charles E. Miller, several students, and J. P. Braselton have collected soil samples from the original site (Timrod Park, Florence, SC) where W. R. Pendergrass collected soil samples for his studies (references below), and Bill also has sent us soil samples he collected. But we have not been able to find Octomyxa on any hosts from soil samples baited with hemp seeds in Emerson's water. Charlie found Octomyxa from soils on the Texas A & M campus when he was on the faculty at Texas A & M early in his career, so we know that Octomyxa is out there somewhere.
References for Octomyxa