There are different types. The thicker type bolt onto the hub, and then the wheel bolts to the spacer. Here are some from Marlin Crawler:
http://www.marlincrawler.com/htm/axles.htm
[Linked Image]

Another type that Allpro used to carry was a 3/4" spacer that used longer studs, but I don't see them on their web site anymore.

Autoparts places usually care thin 1/4" steel or aluminum spacers as well, but you have to check to see if your studs are long enough, and they may not be thick enough to give you the hub cap clearance you're looking for.

'97 4Runner, '06 F350, '86 4Runner, '05 WR450
http://home.4x4wire.com/erik