Where can I get the elastic to repair my patagonia pants with draw up

Where can I get the elastic to repair my patagonia pants with draw up

I have an old pair of pants with an elastic band that draws up. The elastic goes through the pants but is exposed in places as well and has a plastic buckle to draw up the elastic as needed . The elastic is shot and it needs to be replaced. Where can I find the right kind of elastic to replace it?