Ditto the B+Q garden boxes, most people on the yard have them, they are strong enough to sit on which is handy when theres lots of gossip to catch up on!!
IKEA - they sell large zipped square bags in the same plastic/woven material as the blue bags you get to carry round the store with you. They are clear with black binding and handles. I can't recall what they're called but they're only about £3 each. I have loads of them and the zips do go but at £3 each it's not the end of the world. I have 6'9" rugs for my boy and they'll take 2 maybe even 3 outdoor rugs and 3 or 4 thin/medium weight stable rugs/coolers etc. Providing you have deep enough shelving, you can pile them up on top of each other or because they have handles, you could hang them from hooks!
You'll find them in the storage section of the shop - in the Leeds store, they are almost at the end near all the flat packed stuff (not sure if all Ikea stores are layout the same!)
Edited to say: just had quick look on the Ikea site and it's Dimpa Bag