If you wanted to make it yourself you could recycle old sweatshirts/sweatpants (or even t-shirts), assuming you have some on hand or can find them for for cheap. Plus fleece usually comes in basic solid colors at the fabric store so you can get any colors you are missing.
Also, I wouldn't give up looking for a store-bought one yet. I feel like sweaters like this pop up every few years. There are also similar items in Etsy: http://www.etsy.com/search?q=rainbow+sweater&view_type=gallery&ship_to =ZZ&min=0&max=0&page=1
(just be sure you are getting an actual sweater, not a pattern)