Generally known as a Stack Pack or Pack a Main. Normally made to measure by a sailmaker, but if you have a common boat you may well find a sailmaker who has all the dimensions already. All the popular sailmakers such as Kemps, Crusader, Arun, Quay, Jeckells and so on make them, but there are many variations in detail design so best to get two or three quotes.
I had one made by Crusader sails. All they needed was the length of the boom and height of the "stack" - ie the vertical height at the mast from the boom to the top of the sail with the sail lowered.