Something like this is designed to go there. I found mine on eBay, and someone else here mentioned "Tractor Supply". Let me know if you need a link for the eBay one, I still have it saved. it was 6 cost & 6 shipping.
I've seen a caster on a swivel, but:
- I doubt the tongue weight is light enough to move the trailer,
- The trailer is so heavy that if you got it started moving, who knows when you'd get it stopped,
- It's 'taller' than the 'foot plate' and more likely to hit the ground when you don't expect it.
Our trailer did not come with anything either. I bought something called a Flip Jack. It attaches to the jack tube and flips up out of the way to travel when you raise the jack and lowers back down and locks when you lower the jack.