Are you referring to a real world style "Britannia" or is it just a small typo to refer to
WHFB Bretonnia?
Annywho, there are a lot of shields available in the fantasy range and quite a few of them as bits. The dakka swap shop will probably land you a shield for basically nothing as well.
If you're after something more unique, consider using plasticard. With just a little heat it will bend and shape easily. I made four different shields to use in my chaos display base for blood bowl (two of which can be seen here
http://www.dakkadakka.com/gallery/111725-.html?m=2) since I wanted big flat surfaces to paint on.
If I did it for a figure like yours I'd probably embellish it more with maybe a french lily in relief and maybe a crest.