Hi,
All asset store purchase receipts are stored within the Unity account you used at the time of purchase, so simply log in with that account and all your asset store purchases will be available for download.
IF that's not the case, you should contact Unity and explain the case, mentionning what account you used to purchase ( and also forward the email you received at the time of purchase with the order number, it has the subject: "Unity Asset Store purchase confirmation. Order number: ORXXXXXXXXX"
Bye,
Jean