I’m currently in London and I look everywhere for these envelopes.
Can anyone help so that I don’t waste more time in searching? Where can I get those? 
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I would consider looking in a post office, newsagent, Ryman shop or a discount store or WHSmith.
e-bay has them as well as on etsy-plenty of sellers there
I looked at WHSmith but mostly they only have food because they are in a station or very small.
I know that I can get them from online but I meant actual shops 
Try a branch of Ryman, which is a stationery retailer. Either that or a larger, high street branch of WHSmith. The brand of envelope generally stocked is Basildon Bond.
Definitely a branch of Ryman. I think this is your safest option. There are several branches in London.
Also a major Post Office but they will be harder to find and probably not open today.
WHSmith are ‘hit and miss’ nowadays. You could find a big store and they’ll have a very poor selection of stationery.
If you are in an ethnically diverse area of London you’ll probably also find them in an ‘anything’ store - the type that sells all sorts of household goods from buckets to string.
@GeminiFootprints Go to a WHSmiths in a large shopping centre like Westfields and look for the Basildon Bond Airmail letters,