Welcome to the forum, Rubana.
If you let us know your location and the make of trailer I am sure you will get some recommendations.
Without that information the only responses you are likely to get are "go to your local trailer dealers" or "check the diameter of your existing one and look on the web".
DaveS1
Do you mean it has got no clamp, ie a jockey wheel has never been fitted? The A frame needs to have holes to mount a jockey wheel unless you use clamp on mounting. If trailer is lightweight unbraked then smaller size will do job. If it is a heavier braked trailer then you probably need the larger caravan size 48mm shaft.
Thnx everyone for your reply. I dnt knw what the make of the trailer is. But its a big 12ft x 7ft catering trailer. There is allready an jockey wheel fitted which needs replacement. I have seen 1 on Ebay wich is heavy duty 48 mm and is ribbed.
Just saw that there are caravan breakers in our area who sell used parts. I will have a look there tomorrow I might found something really reasonable priced.