Don't you have a welder?
Not hard to fabricate something up yourself, or pay someone else to do it for you if you have too.
I did a search for some pics for you.
Might even make it fold so you can get it out of the way when needed.
Another idea is make a few of these you can roll in and out of your trailer.
Or find one behind a gas station or tire store.
They sell them also but way to pricey if you ask me.
enclosed trailer tire racks