It is rare to find a true (i.e. non-deponent) passive imperative, because the idea of ordering someone to do something is opposed to the idea of having something done to you.
Pinkster, in Oxford Latin Syntax (pg. 164), explains this more clearly:
The grammatical category of 'mood' is one of the means by which the speaker can convey his view of the extralinguistic reality and his communicative intentions with regard to one or more interlocutors. . . .
It is evident that the moods have the semantic function discussed in the preceding paragraph from the fact that there exist cases of (in)compatibility of (grammatical) mood with other entities. For example, in the case of the imperative, it is very odd to order a person to do something when he has no control over it; this explains the very low frequency of true passive imperative forms.
That said, he does cite in section 7.66 (op. cit.) several excellent examples from classical and post-classical sources. He splits them into several categories:
True passive ("rare")
Vis illud augustissimum consulum aliquando tribunal maiestati suae reddere? ascende. Vis constare reverentiam magistratibus, legibus auctoritatem, modestiam postulantibus? adire. (Pliny, Panegyricus, 60.3)
Would you restore its former majesty to the consuls' tribunal, once so much revered? Then mount it. Do you want to strengthen respect for the magistrates, spread restraint among litigants, and confirm the authority of the law? Then let men come before you. (from Loeb translation)
(note that some older editions, such as this one, use accipe and mark adire as a variant in the critical apparatus, presumably because the usage is so rare.)
Quaecumque pestis sive quaecumque es fera, / palam timere (Seneca, Hercules Oetaeus, 1260-1)
Whatever pest you be or wild thing, / be thou openly feared!
Virgil has one example:
Hic tibi certa domus, certi, ne absiste, penates. / Neu belli terrere minis. (Aeneid, 8.39-40)
Here your home is sure--draw not back--and sure are your gods! Be not scared by threats of war!
And (post-classical) the Vulgate:
venite et curamini (Luke 13:14)
Come and be cured.
"Decausative" Passive ("rare")
Lassare et disce sine armis / posse pati. (Luc. 5.313-14)
Grow weary; learn to find life endurable without fighting.
Autocausative Passive ("slightly less rare")
(I read this as similar to the Greek middle voice.)
Tute hoc intristi. Tibi omne'st exedendum. Adcingere. (Ter, Ph, 318)
You mashed up this mess. You must eat it all up. Gird yourself.
Facesse hinc Tarquinios... Devolvere retro ad stirpem... (Liv. 1.47.5)
Be off from here to Tarquinii! Sink back [lit. be returned] into the rank of your family.
Ergo age, care pater, cervici imponere nostrae. (Aeneid, 2.707)
Come now then, dear father, place thyself on my neck.
Sic ergo formare ut scias non posse te consequi, ut sis impenetrabilis. (Sen. Nat, 4a.pr.5)
Be you therefore formed so that you know that you cannot be taken, so that you are impenetrable.
Future Passive Imperative
Used once by Cicero, but in the third person: "apparently intended as a third person plural form, in the style of Twelve Tables."
Regio imperio duo sunto, iique a praeeundo, iudicando, consulendo praetores, iudices, consules appellamino. (Cicero, Leg. 3.8)
There shall be two magistrates with royal powers. Since they lead, judge, and confer, from these functions they shall be called praetors, judges, and consuls.