where can i buy a tv power supply cord?

Here's where I usually go. You have to know what type you need.

http://www.mouser.com/search/refine.aspx?N=19444523

You could try the manufacturer.

From the maker of your TV. They usually have a store for things such as wall mounts, power supply, remotes etc. Or you could try eBay or even Amazon

Walmart.

try a hardware/ electronics store or just go on amazon or ebay, they sell everything (amazon is better btw)