300mm wide insulation

[LancPhil]

Our joists are 1 foot apart and I'd like to lay insulation between them. I've seen it stated in a couple of places that it's possible to get 300mm wide rolls.
I've googled it and looked at a few buiders merchants and DIY stores, but can't find this width. Nearest so far is 380mm at B&Q.
If the right width is available, that would be great - does anyone know who makes it?
It's a very well ventilated (ie windy) loft so I don't want to use loose stuff.
Thanks
Phil