Google Cardboard Ready Made
Since May 2016 Google have been selling their own Official Google Cardboard VR Headsets 
Google cardboard is an open-source project and you can find plenty third party of ready made sets
For longer term use we recommend a Plastic cardboard VR viewer

Here is a small selection of ready made Google Cardboard VR Viewers

Cardboard Cardboard VR viewers
Don't pay more than £15 for cardboard Cardboard
Amazon: v2.0 I AM CARDBOARD = £6.99 (check Amazon P&P)
Google Store: Official Google Cardboard here for £15 or £25 for two (P&P included)

Plastic Cardboard VR viewers
Mattel Viewmaster VR  Amazon = £22.99  Tesco = £20.99 (check P&P for both)
Amazon I AM Cardboard DSCVR = £19.99 (check Amazon P&P)

eBay GOGGLE TECH 3D Glasses  (£16.02 to £30) intriguing portable VR Goggles