Before ebooks and Project Gutenberg, when I bought a paper PD book I always preferred Signet Classics. They were cheaper than the Penguin trades and had far more attractive covers than the similarly priced Bantam Classics. They also tended to have introductions by people you had actually heard of.
Anyway, found a few of the Signet Classics for $1.99 a piece on Amazon:
Moby Dick by Herman Melville
Billy Budd and Other Tales by Herman Melville
Main Street by Sinclair Lewis
Winesburg, Ohio by Sherwood Anderson
There's others, I'm sure. These are the four I bought.