Loft Convertion - Sash window for bathroom and bedroom

[chewymix]

So i am having the loft converted and there will be a bathroom and bedroom, i want to have sash windows in these. Its a victorian end of terrace so the rest of the house is origional sash's. I dont mind modern double glazed units as they wont really be seen but i cant for the life of me find anywhere that seems to offer these units 'off the shelf' and yet all the convertions around here appear to have the same windows fitted.

Does anyone know of where i can get 'standard' sized modern sash's without having them custom made??

thanks