I'm not completely satisfied I've understood everything correctly in this passage, but the sense is clear. Filelfo does believe that the philosophy of "Idea" comes from Zoroaster, but he doesn't cite any writings, just Pythagoras' basis in Magian teachings.
He clearly believes in some kind of "prisca philosophia", but not as Ficino would codify it. There is no mention of Orpheus or the Aglaophemus who was held to be Pythagoras' Orphic teacher.
Filelfo to Dominico Barbadico, 13 April, 1464 (Venice 1502, f. 151v (entire letter, 150r-151v); Hankins 2, pp. 521-22, ll. 250-271 (entire letter, pp. 515-523))
Et huius quidem ideae inventor omnium primus, quotquot aut in Ionica aut in Italica claruissent philosophia, Pythagoras fuisse perhibetur. Secutus is quidem Zoroastren, qui bellum Troianum, ut Plutarchus refert, annis quinque millibus antecessit. Pythagoras enim ut caeteros omnis suae tempestatis, homines, formae venustate, atque praestantia mirifice adeo antecelluit, ut pro Appolline haberetur, ita divina quadam ingenii bonitate atque sciendi studio, cunctis mortalibus superior fuit. Quapropter ubi universam peragrasset Europam, quo undique, quicquid scitu dignum animadvertebat, acciperet, ductus tandem illustri fama sacerdotum, atque prophetarum, Aegyptiorum, profectus in Aegyptum, ubi simul cum lingua omnem illorum sapientiam didicisset, illud etiam intelligere visus est, Aegyptios eximiam omnem disciplinam a Magis, qui a Zoroastre fluxerunt, hausisse. Quare ad Chaldaeos se contulit, quo et Chaldaeos audiret qui astrologiae gloria habebantur insignes, et Magis quos apud illos versari acceperat, congrederetur. Magorum igitur diuturna usus consuetudine non obscure intelligere visus est unum Zoroastren Persen, ut antiquissimum philosophorum omnium, ita etiam acutissimum, sapientissimumque fuisse. Quare ex illa hora, Zoroastris philosophiam amplexus est. Quam postea Plato quoque Pythagoreis usus et auctoribus et doctoribus est secutus. Manarunt in quam ab ipso usque Zoroastre philosopho, quae sapienter et peracute de Idea, scripta a Platone referuntur.
Indeed Pythagoras is held to have been the first discoverer of this idea, of anyone, however many others might have illuminated either Ionia or Italy with philosophy. Of course, he followed Zoroaster, who lived five thousand years before the Trojan War, as Plutarch says. But Pythagoras made himself so preeminent, to all the other men of his time, with beautiful form and outstanding excellence, that he could be taken for Apollo, superior to all mortals, with a kind of innate divinity and spirit of knowledge. He traveled over the whole of Europe, investigating everything that was worthy of attention in its entirety, so that he was admitted, finally, among the illustrious Egyptian priests and prophets, progressing in Egypt, where at the same time he acquired all wisdom in their language, so that he understood that all the excellent Egyptian disciplines which flowed from Zoroaster had been absorbed by the Magi. Therefore he went to the Chaldeans, and heard the astrologers that were held famous among them, and engaging with the Magi among whom he was permitted to dwell. Having enjoyed long experience in the customs of the Magi therefore, he seemed to understand, not obscurely, that Zoroaster the Persian was one of them, as the most ancient of all the philosophers, and thus also the most profound and wisest. Therefore from that time the philosophy of Zoroaster was embraced. Thus afterwards Plato in his turn made use of Pythagoras and was followed by various authors and doctors. They spread, according to what came wisely and acutely from Zoroaster the philosopher himself concerning Idea, writings which are referred to by Plato.
(Filelfo’s Platonic reference to Zoroaster may be referring to First Alcibiades 122a1 :
“And at fourteen years of age he is handed over to the royal schoolmasters, as they are termed: these are four chosen men, reputed to be the best among the Persians of a certain age; and one of them is the wisest, another the justest, a third the most temperate, and a fourth the most valiant. The first instructs him in the magianism of Zoroaster, the son of Oromasus, which is the worship of the Gods, and teaches him also the duties of his royal office; the second, who is the justest, teaches him always to speak the truth; the third, or most temperate, forbids him to allow any pleasure to be lord over him, that he may be accustomed to be a freeman and king indeed,—lord of himself first, and not a slave; the most valiant trains him to be bold and fearless, telling him that if he fears he is to deem himself a slave” (trans. Benjanmin Jowett).
But he doesn't refer to "writings" here, so that's not entirely acceptable. There is another possible intersection of Plato and Zoroaster, when a philosopher named Colotes wrote that Plato had changed the name of Zoroaster to Er in Book X of the Republic. But again, no books are mentoned. Maybe Filelfo's reference is to some apocrypha we don't know about.)