is included the power supply PWR-2821-51-AC
in the CISCO2821-SRST/K9 Part Number?.
CISCO2821-SRST/K9 comes with the Power Supply you mentioned by default. You just need to select the appropriate power cable.
The Cisco 2821 chassis comes with the power supply included. Not sure if you would get the same part number mentioned above. Looks like that part would fit in for a 2821 and 2851.
Hope this helps