You can get walk tickets from over the counter at the Windsor Information Centre.

Walk tickets – The Windsor Information Centre:
You can buy tickets over the counter at the Windsor Information Centre They are a very friendly team and you can also get information about other attractions in Windsor.
Royal Windsor Information Centre,
The Old Booking Hall,
Windsor Royal Shopping,
Thames Ave,
Windsor SL4 1PJ
Hours: 9AM -5.30pm Monday – Saturday
10am – 4pm on Sunday
Map: here
Payment:
Royal Windsor Information Centre. Accept major credit and
debit cards except American Express and Diners Club. Card payments will
appear on customers’ statements as Windsor Information centre. Cheque
payments made payable to Royal Borough of Windsor & Maidenhead.
Cheque must clear prior to event. Allow 6 working days.
Telephone Tickets:
You can also purchase tickets over the telephone: 01753 743900
Click to see more images here.