" The Widow Douglas she took me for her son, and allowed she would sivilize me; but it was rough living in the house all the time, considering how dismal regular and decent the widow was in all her ways; and so when I couldn't stand it no longer I lit out. I got into my old rags and my sugar-hogshead again, and was free and satisfied."
The alliteration is found in the phrase "free and satisfied". The repetition of the "f" sound adds a sense of emphasis and rhythm to the sentence, highlighting Tom's sense of liberation.
There are other examples of alliteration in the book, but this one is particularly notable because it emphasizes a key theme of Tom's character: his desire for freedom and adventure.