The best ones I've found for my cocker are the Pawz ones (
http://www.amazon.co.uk/Pawz-Waterproof-Dog-Boots-Medium/dp/B001J1Y9VA/ref=sr_1_1?s=pet-supplies&ie=UTF8&qid=1378486019&sr=1-1&keywords=pawz+dog+boots). They are really waterproof, last very well on rough ground (even though they just look like balloons), and come in a range of sizes. They also stay on when he runs off lead. We tried some of the ones (sold in pet shops) which fasten with velcro, but they were forever coming loose or falling off, and lasted for only half a dozen walks at most.
My only reservation with the Pawz ones is that the neck of the boot which goes round the leg is very small, so I make a small snip with scissors to make it more comfortable for my boy, which compromises the life of the boot - each one lasts about a week, but they come in packs of 12 so I'm not too worried. HTH.