Those take standard IEC power cords (IEC is the name of the connector on the amp; the connector to the wall will depend on what region you're in). Very common cords. In the US you can buy them at Monoprice, Amazon, Best Buy, Parts Express or a zillion other stores. Try to get one that is 14 gauge or even 12 gauge. A 16 gauge one would be a little weak but not terrible for the 800. Do not get an 18 gauge one. Lower number for gauge is better. Some stores just call them "power cords" and don't say IEC but usually they will be labeled IEC.
Here's an example - you can buy this exact one, or find a similar cord at many other stores https://www.monoprice.com/product?p_id=5279 However, make sure to click 14AWG in the item details. I clicked it before I copied the link, but the link I just gave you still seems to default to 18 AWG.