I don't know about big box stores but I did want to mention padding. Years ago we put carpeting in our bedroom and in an effort to be economical picked a thin pad. I've regretted it ever since. I have another room with a thicker pad and it feels so nice and cushy where our bedroom is rather flat. I think part of it is the pile of the rug too as that rug does have a lower pile.
Something else to consider, if the room is not too big, is remnants. Flooring stores have "remnants" that are big enough to be wall-to-wall in some rooms (or maybe need one seam in an out of the way place). The color choices may be limited but worth a look. I think one room we did that way is about 12 x 14.