Cracked Rim-what now?

Your rims came from Tredit. They are guaranteed for life. That rim is no longer made. If you call them they will replace it (and usually it's neighbor to preserve the same look) with an option to buy 2 more (for the other side) at a reasonable cost. Not an uncommon problem described in detail on the free to join Montana Owners Club web site (montanaowners.com)

Might think of buying a matched pair of the same size if you can't match what you have. Unless you point it out nobody looks at both sides of the trailer at the same time...

Once you take the wheel off the trailer you might find all the info you're after about the rim.

Something like this stamped in.

You have not provided any description of the rim (steel, alloy, or?) or the damage. (pics) Under some circumstances, if not available, rims can be repaired with a short turnaround, (< days) but that should ONLY BE DONE BY QUALIFIED PEOPLE WHO KNOW WHAT THEY ARE DOING. References required. As others have mentioned, buying a new identical rim is your best option, if possible. Google is your friend.

Chum lee

Typically the spare would be the same sort of wheel as the rest, so it could be used indefinitely if the tire mounted on it is fine. This isn't like many cars where the spare is a temporary use donut; and even in those the tire rather than the rim is the main limitation on how long one can safely use it.

Spare can go 10s of thousands of miles.

If you post a picture of the wheel, tire size and maybe the manufacturer... somebody will probably identify where to get one fast.

https://recstuff.com/trailer-wheels

Try Carid

I've bought rims from them a couple times, and had no complaints.

Look at the rim closely. Normally on the inside is the manufactuer.
Measure your rim for width and bolt circle.
Might check etrailer to compare against what you have.