That's entirely up to you; wear whatever you are most comfortable in. Remember that, depending on the time of year, the weather may be cold and/or wet. Most people wear walking boots although some prefer wellingtons. Waterproof trousers, or overtrousers, are also an option. You could wear shorts, but remember that there may be stinging nettles. Since we often go into pubs, something to go on over your boots would also be a good idea (a couple of carrier bags will suffice) as landlords are not all that keen on clods of mud on their carpets.
If you look on the main menu under Links | Clothing, equipment and accessories | Medisave 16" plastic overshoes, you will be taken to a website from which you can buy them direct. Click here to visit the site. The only problem with buying from there is that you may have to buy a large number. I have bought some, which I sell on at cost. It currently works out at 25p for 10.