Unlike many of Shakespeare’s plays, All’s Well That Ends Well doesn’t seem pilfered from multiple sources. No, it looks like Willy only stole from one author here.
In the mid-fourteenth century, Italian writer and poet Giovanni Boccaccio wrote what is considered to be his masterpiece, The Decameron, a collection of stories, told from the perspective of ten characters who each told ten stories apiece (deca is the Greek root for the number 10, natch). Think of The Decameron as the Italian Canterbury Tales… before there was The Canterbury Tales.
Now, Shakespeare–as far as we know–didn’t read Italian, so it is most likely that he got his Decameron via an English translation of this particular tale by William Painter in his collection of stories, The Palace of Pleasures, the second edition of which appeared in 1575, just before Shakespeare began his writing career.
Painter’s summary of the Decameron’s ninth story of the third day contains a story that sounds remarkably like our play:
Swap out the name Helena for Giletta, and tweak Beltramo into Bertram and Rossiglione into Rossillion, and you’ve got more than just the broad strokes of All’s Well That Ends Well.
[Fun fact: Boccaccio earlier in his career wrote a poem called Filostrato, which became the basis for Chaucer’s Troilus and Criseyde, which in turn became one of the sources for Shakespeare’s Troilus and Cressida.]