Hi Paul.  Have you tried the sizes they do have in stock?  I think the general rule is, ballet shoes, one size bigger than your shoe size.  But this isn't alway the case.  I'm a size 9 but have found some brands fit perfectly at size 7!!  Other brands have had to be two sizes above my shoe size.  Ebay have a good selection of canvas split sole (brand new) going up to 10.5.  They might be worth trying as 10.5 could be the equivelant to your twelve.  Ballet shoe sizing I think is extremly vague.  Try different stores and ask to try their largest sizes.  Don't let them put you off by saying "a 10 is the biggest we've got.  That won't fit you".  That's the attitude I came across, "your a 9, these 7's will be too small".  Well my friend they weren't.  A lot of these stores treat us blokes with suspicion and can't wait to get rid of us.  Stick by your guns mate.

Good luck,

         DannyUK