If you no longer have any documentation with your customer number on, you have 3 options.
1. Email the address that Saint Stevo gave.
2. Telephone (long waits).
3. In person at the ticket office.
If you are intending to buy your tickets over the phone or in person, you don't actually need your customer number, your name and address details should suffice.
They will be able to give you your customer number at the same time.
(Besides....... I am NOT a number, I am a free man etc etc.)