Neck rests leave your head free to look around, bob for balance and leave your helmet alone.
So, most head rests are really actually neck rests.

Yes, you need one.

Mr. Pedals likes to keep his shoulders ahead of the rear axle and that's superb advice.
I must keep my inner ear inside the axles.
When I've tried to ride my bike with the balance-sensing organs in my head behind the rear axle... well, balancing gets kind of fun.
But that's just me.

Google is your friend, my friend.
I can't/won't help you find a head rest as I feel it's a very personal choice, much like your helmet, gloves or shoes.