The 2500 is the Roku HD released 'way back in 2012. It uses a 12v power adapter with a permanently attached cord that is available from the Roku accessories page (https://www.roku.com/products/accessories/player) for $9.99 -- scroll down to the PW01 adaptor. You can also find them from a number of third parties on several sites like Amazon and Walmart -- do a web search for roku 2500 power cord.
My advice would be to junk the 2500 and get a current unit. Roku considers the 2500 to be legacy equipment that is no longer supported and is no longer receiving updates. It lacks features of current models and by today's standards is extremely slow. An increasing number of the more popular channel apps cannot run on it and aren't even shown in the 2500's Channels list for download. A replacement power supply for the 2500 will cost 1/3 to 1/2 the price of a current more capable Roku model that comes with it's own power supply.
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