What subwoofer do I need for an 05 Acura MDX?

I blew the woofer in my mom's MDX, she's selling it soon so I just want to get a cheap replacement to get it by until it's sold, nothing fancy. I think it's an 8" but I'm not sure where to look though. And I need it SOON so an actual store would be best.|||Take out the one that is there and measure it with a ruler. Or just take it to the store and ask them what size it is.