I have done a search as well without any success so maybe the rail trim end caps could now be obsolete. However, if you state the year of the caravan it may possibly help with your search.
Have a look on the Leisureshop direct website. They have lots of spare parts or you can email their technical department to ask. They usually reply pretty quickly.
I looked on both 'Leisure Shop Direct' and 'O'Leary's' websites plus an extensive search on Google images and couldn't find anything that looked identical to the end caps you are searching for. However, you have done the next best thing by contacting them with the corresponding photo and hope you get the right replacements you need.
If that fails then the only option available will be to replace the complete rails of a different design with the corresponding end caps.