The Lewes Town and Country Bed and Breakfast Group http://www.lewesbandb.co.uk/ has a list of reasonably priced B&B accommodation.
A new B&B has just started up and is not yet in the listings - Orchard Bed and Breakfast, 22 St Annes's Crescent, Lewes, BN7 1SB Tel: 0800 783 7997
Spring Barn Farm Park have kindly agreed to open their campsite for the festival. They are about 1 mile from the centre of Lewes.
Contact them directly via their website.