http://madrobotminiatures.com/zencart/index.php?main_page=product_info&products_id=17
http://www.pig-iron-productions.com/hd24-berets-p-85.html
The beret probably isn't an exact match to a caubeen, and needs a hackle added, which you could do with plasticard or green stuff.
It would look all right in 28mm, though, unless people have a magnifying glass.
I would be a bit worried that the hackles would snap off easily from being the most likely point by which the figures will be picked up.
Another option is to get some heads, modify the beret and hackle with putty to make a master, and mould them in resin. I'm not sure about the copyright position on that -- you might need permission from the original head modeller.
Hasslefree Miniatures do separate heads for converting and you are allowed to make castings from your piece. There would be more work involved in making the hat and hair, but maybe not that difficult. (Actually it might be interesting...)
http://www.hfminis.co.uk/shop?category=accessories~converting