You’re correct about the PC-27BL being the power cable for the 525. Just looking at the small picture of the PC-26BL I can’t see any differences, but the pin configuration in the plug may be different between the two. Other than that, the three pigtails coming out of the plug appear to be the same….one for network power, one for graph power, and one for NEMA communications.
I just installed a 520 and it was the same thing and the cable came with the unit. Your’s should have also, but that picture of the PC-26BL should be close enough to know what you’re looking for.