As one of its daily deals, Hobo Ninja offers this Spock / Leonard Nimoy Tribute Decal for $3.99 with free shipping. That's a buck under the lowest price we could find for a comparable decal elsewhere, outside of the mention below. It measures 3.5" x 3.8". Deal ends March 4.
A close price: Redbubble offers a similar decal for $2.40 plus $1.87 for shipping.