At air museums all over the country: March AFB in Riverside, plus several others already mentioned in this thread. You can walk right up to, and underneath, them, although I doubt any museums allow you to climb into the cockpit--March doesn't.
In Virginia is a big aerospace museum. The Enola Gay is there, as well as a Concorde.
I hopped the velvet rope so as to put my hand on the SR-71, only for a second.
I was truly honored.
The country that made the SR-71 no longer exists, and there is nothing we can do about that, except take names so as to punish those responsible and see to it that it never happens again.