Where can I get a rain cover for the duet?
The rain cover should have been included - or it usually is. You can get the rain cover from the seller you got your pram at, or ask any seller of this model/brand to order it for you. The last thing to try is also to get it from the manufacturer's customer service (firstname.lastname@example.org).
Also, try considering any kind of double (side-by-side) stroller rain cover that fits universally, e.g. on amazon.