As you can see the finished stitched area using 14ct aida is 17.9 x 22.3 inches. In order to have enough fabric for framing you should add another six inches in each direction. You need to have fabric that is 24 x 29 inches

Lovely pattern! :)

I just realized that I did not answer your question. You asked where to order it and I told you the size you need. You cannot get a custom cut here. You can buy a tube that is 30 x 36 inches and cut it to the size you want.

Click "Cross Stitch Fabric" in the box on the left side of the screen. That will give you some options.