It's surprisingly hard to get hold of a real swag in the UK. I inherited my Razorback swag from my wife's family when we were living Down Under several years ago: that's the brown one below and shown in our main post about swag-camping here (and you'll see it pop up all across this site as it's been my main outdoor accommodation for ages now - I haven't used a tent for 7 years...).
There's a lot of swags available on Ebay, from Australia see here http://shop.ebay.co.uk/i.html?LH_AvailTo=3&_nkw=swag+canvas - worth a look but the postage is pretty heavyweight - but great for looking at the real Aussie swags that are available over there: lots of different shapes and sizes.
Update: Another company based in Devon also sells Australian style swags - they are made in the UK unlike the imported ones above, but are based faithfully on the original Australian designs - see http://www.wruffit.co.uk/ for details:
[Edit: Seems that Wruffit are no longer trading... website has been down for a long time and no answer on their phone number - so one less place to buy a swag now!]