I had visor replaced under warranty on my '03 - price was $153 just for part! I'm sure Yota doesn't sell just the mirror but you could probably cut and cobble one in as a replacement. Or try ebay or junk yard.
www.Partznet.com offers a variety of collision and replacement parts. I buy parts for my 350Z and 4runner from this place. Other members here have also had good business with them.
Here is a link to a replacement glass for the driver side mirror.
Since i'm not sure what model your 4runner is i chose the mirror with heat option on the search but its easy to navigate through.
oopsie i think i made a mistake referring to yoru mirror haha :bash: . My mind is just confused today But if its the sunvisor your looking for they run for about $49.07. These are replacement visors and i can't find a replacement mirror anywhere else in the catalog. Here's a link to it
i'm not to sure if its it, but you may want to call to verify if its the part.