They're not to be found anymore. Apparently all the Panzerfaust sights have dried up...
The others mentioned don't seem like they'd work, unless someone knows a flip-out magnifier other than LaRue because the LaRue ring doesn't actually open up.
On arfcom there is a thread on how to make your own magnifier out of the hensoltd magnifier also sometimes you can find the larue ones in the classifieds.