Cosyndry are the best online place I've ordered from, the woman who runs it is really friendly and helpful, and they deliver in 48hrs. I wholeheartedly recommend them.
On the high street your choices are virtually non-existent. Boot's do the infamous Staydry nappy, which is awful and a waste of time, money, etc. I'd see if there's a medical supply shop nearby, these usually stock assisted living products, and that includes adult nappies. Look in your yellow pages or online for Medical supplies, and then you'd probably have to give them a call to see if it was worth a trip over. In fact, I think there was a thread on here listing all the places you can go in the UK, but as there aren't that many so it died rather quickly. I'd look it up but I can't be arsed.
Most pharmacies will allow you to order them in, too, but this is a right pain, and combines the embarrassment of going into a store and talking to another human about adult nappies, with the inconvenience of waiting for an order to show up.
I'd just order online. TBH