People that choose to sleep on their back or stomach at night and stay that way all night long are very rare indeed. Most of us may start out on our backs, but before long we end up sleeping on one side or the other. Sometimes we flip back and forth from side, to back, to the other side!
It really is a natural thing to flip around while we sleep. One part of our body can get cramped so we flip to another position. This can really play havoc with your pillow, especially if you are constantly reshaping it during the night to make it adjust to the new position you've just moved into.
You need a tough pillow to hold up to all of that tossing and turning or you could end up with neck pain. The best choice is going to be a pillow made with memory foam. Now, there are two types of memory foam, shredded and solid. If you like to bunch your pillow up into the shape of your choice, then you should go with shredded foam, but it will still pop right back into shape when you release it. If you like a uniform pillow shape, then you should choose the solid foam.
As to who sells the best memory foam pillows for side sleepers, I saw many different reputable brands at http://luxuryofthepharaohs.com/side-sleeper-memory-foam-pillows/. It's always best to compare different options before you make up your mind to go with one brand or another, especially when it concerns something as important as your sleep!