There is a large number of letter size guides, some times known as postal rate guide, available to buy.
It may be possible to get one for free at your local Post Office but if not then below is a list of suggested places.
These will not be free and as in so many products the choice for letter size guides are varied in price as well as quality.
The best one to buy is the actual Royal Mail Size guide version which can vary from £12 – £30 or more. But there are letter size guides that retail at a few pounds. These are usually made from plastic…..similar to school rulers and therefore can break fairly easily.
Listed below is what is currently available on Ebay and Amazon UK.
Please read the reviews of the letter size guides that are for sale from these to major retailers. Buying from Amazon and Ebay these days is fairly safe and just need you to use common sense, If the seller on Ebay has less than 99% good rating move on to the next seller. Likewise for Amazon. Always pay by credit card so if anything does go wrong you are protected by your Credit card. (See your own credit card company for details).