Readily available in stores: In the UK, there are two options: bed wetter nappies or adult/incontinence nappies:
Drynites -- These are pretty much larger versions of the old style Pull-Ups. Available in the US under the 'Goodnites' brand. The girls version is better for any boy who has gone through puberty, because of where the landing area is. They are apparently also slightly stretchier.
Cost: 9 for £5.34
Available at: ASDA, Sainsburys, Tesco, Boots, etc.
Freebies: Free sample and £1 off coupon
Adult Nappies: Here the market is dominated by Tena, Poise and Boots' "StayDry".
Tena:
'Pants' -- The Discreet version of these are available in most Supermarkets, Chemists and Boots. They're not particularly absorbent, one wetting, two at a push... and they're not particularly discreet either. Not really that good. Eats up odor quite well though!
Cost: 12 for £7.50
Available at: ASDA, Sainsburys, Tesco, Boots, etc.
Freebies: Free sample and £1 off coupon
'Slips' -- These are found in the pharmacy at ASDA, a large branch mind you... and they're quite expensive.
Cost: 28 for Approx £25 Available at: Larger ASDA's, with large Pharmacy