Hi - it's a rare size and you will probably have some trouble replacing like for like.
However, the next ISO size up - ISO451 will fit the bike just fine (just 5.5mm bigger at the brakes), and a resurgence of popularity in this size means you could either source an old rim/SA hub combination from the UK contemporary of the Kingpin - A Raleigh Twenty / Shopper / Stowaway (also badge engineered as BSA, Philps or Triumph) or get something bang up-to date built up with a new SA hub and lightweight Alloy 451 rim from a maker like Velocity or Mavic.
[Edit - I just saw your location - if you do source an old Raleigh bike wheel be sure to check it's not got ISO 406 wheels (Most N. American ones do). These will also fit but lower your bike by about an inch and may lead to issues of pedal strike, as well as possible issues in your brake calipers reaching the rims)