Here's a travel blog with some more info and pictures of Sheep Creek:
http://www.maryjomellado.com/Camping...sequoia_01.htm
(Be sure to check the other "camp" links on the left of the page to view more site photos.)
This includes how to pay/"reserve" your site upon entry. However, sites are now $18 per night, according to the official park site (that info is on the "Key Information" page in the Wiki).
Tip: for those who want to save extra money, try to plan on the maximum of 6 people in your site (maximum of 2 cars) so you only have to pay $3 per person per night. 5 people in a site runs $3.60pp, 4 people at $4.50pp, 3 at $6.00pp, and 2 at $9.00pp. And, sometimes, that extra space is worth the extra money, so it really depends on personal comfort level.