Toyota doesn't appear to sell just the cover, they'll only sell you the whole lid. Part number is: 58905-35140-Cx. Where x varies by trim color. But it's spendy.
The only thing a cursory Google search produced was this?
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"Sometimes I sit and think. Sometimes I just sit."
Canvasback makes very nice covers for the armrest, as well as the cargo area, wheel wells, etc. Their dark fabric with red thread is a great match for my 4Runner interior. Very high quality.