I am trying to think of what US brewed beer tastes most similar to a Weihenstephaner Hefeweissbier...
You can find a discussion about it here.
A couple of US brewed beers that arise from it are Sierra Nevada Kellerweis and Sly Fox Royal Weiss; but if you'd prefer a more easy-going (and usually cheaper solution), I'd go with Gösser Dark - although it's not make in the US (but in Austria) it's widespread across the country.
Erdinger Urweisse and Erdinger Hefeweissbier should also be close. Both are brewed in Erding (roughly 15 miles from Weihenstephan), but reportedly available throughout the world.