As far as I know, you can't. There may be a new tetanus-only vaccine, but I've only seen it mentioned once in a post here.
Tetanus is VERY difficult to get. I researched it thoroughly, because it's one that I was worried about. First, the bacteria are anaerobic - they can't live in the presence of air. You know how everyone always goes on about how if you step on a rusty nail, you'll get tetanus? NOT SO. Anything sticking above the ground (in the air) cannot contain live tetanus bacteria. Furthermore, simply washing a wound usually removes bacteria. And a child's circulatory system is so good, that children almost never got tetanus even before the shot existed. To get tetanus, you would have to get a deep puncture wound from something that was buried underground (or lodged in feces), and then not clean it.
Rust has nothing to do with it at all.
I hope this helps!
Mom to Jackson, b. January 2006
and Cassandra, b. October 2011