Have you looked here yet?

http://mrtruck.net/popup.htm

They have a pretty large assortment to choose from.

http://uniquehorsetrailers.blogspot.com/

Before you merely consider price... There are two types of GN extenders.

B+W and Pop-up's.  Each has advantages in moving the trailer connection.

Pop-up's keeps the hitch weight forward, but requires swing clearance.

B+W moves the hitch weight to the rear but doesn't swing in the truck bed.  This could be a consideration in rear axle loading.

Both B&W and Popup have 4" ball extensions, which I quit using. It actually put enough leverage on the hitch under the bed to loosen the bolts on the hitch center section. 4 inches doesn't give enough extra swing. Popup has 9 and 16 inch gooseneck extentions. Convert-A-Ball has a gooseneck extension also.

B&W had a 7 inch Extend-A-Goose coupler gooseneck extension that was recalled by B&W and reported to the NHTSA for cracking welds. It's old news from 2-26-2010, so hopefully all the buyers of the extensions have been notified.  

http://www.allworldauto.com/recalls/NHTSA_10E004000__B_W_EXTEND_A_GOOSE_COUPLER_recall_103262.html