How can I write a formalist thesis about "A White Heron" by Sarah Orne Jewet? I have to find two outside sources about formalism and apply formalist thinking to the text.

Formalism is a school of literary criticism that focuses only on features of the text, including its syntax, grammar, motifs, and tropes, rather than on the biography of the writer or the historical context in which a work was written. Formalist critiques of "A White Heron" include that by Smith (1983; see the sources below), who examines how the language that Sarah Orne Jewett uses helps express a connection between people and nature. Hovet (1978) analyzes the structure of "A White Heron" and looks at the "morphology of the fairy tale" to understand the literary structure of the story. Plourde (2011) looks at the symbols of the text as a way of understanding the way in which Sylvia, the main character, rejects a heteronormative world. For example, she looks at phallic symbols in the text, such as the gun the young hunter carries in the following passage:

"She did not dare to look boldly at the tall young man, who carried a gun over his shoulder, but she came out of her bush and again followed the cow, while he walked alongside."

By rejecting the hunter's demand to lead him to the white heron, Sylvia, the main character, rejects the heteronormative world for a world of women. As she hides from the hunter, she basks in the roundness of the sun: "Yes, there was the sea with the dawning sun making a golden dazzle over it." These symbols represent her turn away from the world of men and women towards the world only of women. Your essay could focus on one of these symbols, such as the gun or the sun, and use the meaning of this symbol as your thesis. You can look for textual evidence about the way this symbol is used in the story and its significance. 
Sources on formalism:
Shklovsky, Viktor. Theory of Prose. Trans. Benjamin Sher. Elmwood Park: Dalkey Archive, 1990. 
Trotsky, Leon. Literature and Revolution. New York: Russell and Russell, 1957.
Wellek, René, and Austin Warren. Theory of Literature. 3rd. rev. ed. San Diego: Harcourt Brace Jovanovich, 1977.
Sources on "A White Heron"
Hovet, Theodore R. "'Once upon a Time': Sarah Orne Jewett's 'A White Heron' as a Fairy Tale." Studies in Short Fiction 15 (1978): 63-68.
Plourde, Aubrey E. (2011) "A Woman's World: Sarah Orne Jewett's Regionalist Alternative," Rollins Undergraduate Research Journal: Vol. 5 : Iss. 1 , Article 9. Available at: http://scholarship.rollins.edu/rurj/vol5/iss1/9
Smith, Gayle L. "The Language of Transcendence in Sarah Orne Jewett's 'A White Heron'." Colby Library Quarterly 19.1 (Mar. 1983): 37-44. Reprinted in Nagel, Critical Essays on Sarah Orne Jewett.