Whilst not exactly a whole rainbow, this fabric is from the Rainbows collection, and it is a simple dots design in autumnal shades. 

Sold by the metre and available in 0.25m increments. If you order 0.25m you will receive it as a fat quarter, but all increments from 0.5m and above will be sent as one continuous length. If you would like it cut differently, eg. a long quarter or 0.75m as 1 half metre and 1 fat quarter, please do leave a note in the comments.