These shouldn't be hard to get. 27 x 1 1/4" tyres are still available at most price points from about £4 and upwards (look on
ebay). For a 1950s road bike, I would say the Raleigh branded tyres are probably the most authentic you'd get today.
There are several sizes of 26" tyres, all of which aren't interchangeable. 26 x 1 3/8" are easily available, again from about £4 and upwards, depending on how much you want to spend. Again the Raleigh Record tyres would look authentic on a '50s bike..
26 x 1 1/4 isn't so common but I have a look on
www.sjscycles.co.uk as I think they have stock.