You may sign up for a Zero1 Jumbo plan at the following:
Zero1 Customer Service Centre located at 8 Burn Rd, #01-03 Trivex, Singapore 369977
The Zero1 website or Zero1 Mobile App
Over 70 authorised retailers across the island listed here
Accepted payment methods:
Credit/debit card (website or app sign ups ONLY)
Cash (Customer Service Centre or authorised retailer sign ups ONLY).