^^^ Above is called a 'Sandskipper'. They're an option on Indespension trailers, at £99 plus the wheel, so not inexpensive. However, if memory serves, they were manufactured by a company in Liverpool. Most Indespension dealers would be able to supply you one.
They swing down and act as a jockey wheel, for use over soft ground, where a normal jockey wheel would dig in. However, there's no ability to steer once deployed. Also handy as they keep an 'emergency' spare wheel and hub on the trailer at all times (in case you throw a bearing / get a flat whilst towing)
Google/Bing 'Indespension Sandskipper' for some feedback.
Look like this:
But if you just wanted one that was stationary on the trailer then you could easily fab one up on some plate, using a stub axle.
All the best.