Changing from a regular bed to a Shikifuton. Problems

Hello.  I do not know if  anyone will have an answer.  This may be too specific a question.   I am purchasing a Twin XL shikifuton, 39" x 80" x.5"  I am 6'1" and live in very limited space, which is explains my choice of futon size.  First, how do you find sheets for futons and then how would I find sheets that would fit this size of shikifuton.  I have tried looking through Amazon Japan and have had very little luck there. Are there specific Japanese words I could use to search on the web or certain stores in Japan that would be most likely to have something like this?  I am willing to pay to a certain point if I have to have sheets modified.  I just hate to think that I will just have to use oversized sheets that never fit my futon.  Any help would be appreciated.