Have you tried emailing the company direct to see if they can help you out? They might have something in a warehouse somewhere? Might be worth a try?
The only other thing I can think of is the Tetra Tea Tree products, I think they have a head support?
Yep just checked - found them on the basically baby website, but I thought they had their own website as well? Anwyay, if you go to baby accessories, there is a baby headrest. Not sure if this would be suitable or not though?