If you want to use ready to roll marzipan you won’t find it in the local supermarkets where you would expect to.
It’s not with the cake/baking ingredients. You’ll find it in amongst the dried fruit and nuts section. Its called Pate Amande and it comes in a small block of 250g, normally in 3 colours (pale (ish) green, cream and pink, so icing should hide it) .
It is on special offer in Intermarché currently and costs 1,30€ a pack.