Without any further information about those sites, only you can make the determination as to whether what they say is reliable.
Did you have any place in particular in mind?
I was looking in Google for a couple of genres of mp3 free downloads (Christmas and patriotic).
If a site says that it "guarantees" that the artist(s) has given permission, is that reliable?
This music is "old standards", and beyond.
The iTunes free site does not have such a selection.