Hi
You could try
www.conservation-by-design.co.uk
Look under 'Cloth covered boxes' and then 'Timecare Museum Portfolio Boxs'
It says that they will make one to order in a colour, size to suit your needs. One things for certain, It will be superb quality, and they will treat you with the upmost respect, whether ordering one box, or 1000 boxs. I can't vouch for their delivery etc though, as I collect all my goods from them.
One other alternative is to Optically centre your print in a rectangle mount. It looks very attractive and all your prints are in the same mount/frame size if your a user of other formats.
Hope this helps.
Regards
Stoo