The easiest way to get a disc is to call your Toyota dealer's parts department. They probably have the new disc in stock. Toyota issues a new disc just about every year. It'll be a couple hundred bucks. You can probably find them on eBay, etc., but I'd be a little wary of a non-OEM disc.
I'm not sure where you install the disc in your model, but some typical places in other models are behind the nav screen, in the glovebox, or a module in the trunk. If you can figure out where the module is, it's an easy install, maybe all of 5 minutes: turn on the car, take out the old disc, put in the new disc, wait until the screen says it's done loading. Simple. If you're nervous about messing around with the software, have the Toyota guys do it. You'll probably get charged an hour labor though.