Huldreich (or Ulrich) Zwingli (1484-1531) was one of the founders of Protestantism. Note: since he rejected the orthodox Christian doctrine of original sin, he cannot be said to have espoused Mary's immaculate conception, because that doctrine presupposes original sin (in order to remove it from Mary by grace).
[Zwingli's own words are in blue]
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1) Hans Joachim Hillerbrand, Encyclopedia of Protestantism, Volume 3 (Taylor & Francis: 2004).
Although Zwingli did not explicitly state a belief in Mary's immaculate conception, he did emphasize her sinlessness and her role in the stainlessness of Christ's conception. (p. 1173)
2) George Henry Tavard, The Thousand Faces of the Virgin Mary (Liturgical Press: 1996).
Although he does not explicitly relate Mary's virginal and immaculate conception of her Son with her own immaculate conception, Zwingli does call her "immaculate." As he also wrote in De vera et falsa religione, she was without "the smallest trace of a stain." (p. 107)
3) Gottfried Wilhelm Locher, Zwingli's Thought: New Perspectives (Leiden: E. J. Brill: 1981).
Zwingli goes so far as to state: "I firmly trust that she is exalted by God above all creatures of blessed men or angels in eternal bliss." [Z I 424; H 1 159] (p. 88)
. . . forceful expressions which Zwingli frequently used to describe Mary's purity ("immaculata", "illibata", "purissima", etc.) . . . [Zwingli:] ["]God has also sanctified and purified the mother (of the holy Son), for it was fitting that so holy a Son should have so holy a mother.["] (p. 88; original Latin version is also documented on this linked page)
4) Raniero Cantalamessa, Mary: Mirror of the Church (Liturgical Press: 1992).
In a sermon in 1524, Zwingli called Mary "the pure virgin Mary, mother of our salvation," and he stated that where she is concerned, he never "thought, let alone taught or publicly affirmed the slightest thing that could be impious, dishonoring, unworthy or bad of her." (p. 130)
5) Andrew Pettegree, The Reformation: Critical Concepts in Historical Studies (Taylor & Francis: 2004).
. . . the eternally pure body of Mary . . . (p. 287)
6) Donald G. Bloesch, Jesus Christ: Savior & Lord (Downers Grove, Illinois: InterVarsity Press: 2006).
Zwingli could refer to Mary as "the Mother of God, the perpetually pure and immaculate Virgin Mary." (p. 117)