for sale is a pair of Oxford panniers. these are surprisingly big and can carry plenty as they're expandable too.  they also have a couple of useful side pockets and come with waterproof covers that pack away into their own little pockets at the front of the bags.  They fit over the back of your bike seat, with one of the velcro straps supposed to go under the seat, with the other two over the top.  They've been fine like this for me, but if you're nervous you can also secure them with bungees as there are additional hooks.