SPI Supplies offers its own "brand" of HOPG. It comes in three different grades as well as several standard sizes.
Grade SPI-1 is very similar to Grade ZYA described below and is comparable to the very best "calibration grade" HOPG. Grade SPI-2 is very similar to ZYB, is slightly less highly ordered than SPI-1 but is acceptable for most users. Grade SPI-3 may not be acceptable for the most demanding of experiments but for student use its much more economical pricing makes it ideal.