Bought some from eBay mate, it's really cheap.
http://www.ebay.co.uk/itm/112491443839
The existing loose elastic is quite thin so the one I used was 3cm width. I took the advice from the guy in that video.
I'd say 1m in length is more than enough to order. I doubled up on both sides and stapled.
Stitching would be more professional and robust but when you start working on it and see how awkward it is you soon realise why stapling is easier.
Hope that helps.
Beans