Possibly the old RTB01 (beaky) heads?
Otherwise the smallest ones I have found are from Anvil Industries, they do an unhelmeted range that you can imagine actually physically fitting inside a marine helmet, and are therefore a little smaller. I had a similar issue previously trying to fit a head inside a very narrow neck area and ended up using these:
http://www.anvilindustry.co.uk/index.php?route=product/category&path=77_66