I have a trailer (or trolley) nose wheel for sale for £20 ono. I can post at cost if required.
It is a heavy duty model in galvanised steel with a solid rubber wheel diameter 10 inches, width 2.5 inches. It comes complete with a mounting bracket that you can clamp onto a 50mm square section/arm of your trailer. There are two U clamps if you would like them for this purpose.