i would not buy one over the internet. but if you go to your local comet, currys or pc world they have a wide selction and you can try one to see if it fits and if not return it.
If you are looking for a back pack, have a look at Tech Air. They have an inner sleeve that's surrunded by a sort of bubble wrap. The bags come with i-Track which is a recovery/finder service. The bags vary in style from groovy to formal and everything in between.
I've used mune for 2 months commuting through the London crush hour on tubes/buses/trains and it works! Mine is the 5701 and it cost me about £40. Good luck
