The type of bearing cap depends on the type of axle. If there is a grease zerk on the end of the axle, it would be something like this:
https://www.etrailer.com/Trailer-Bearin ... -DCRP.html If there is no grease zerk on the end of the axle, you probably need a "Bearing Buddy" style like this:
https://www.etrailer.com/Trailer-Bearin ... 1980A.html The links are from etrailer.com, but you should be able to find other local and online sources.
You may also look at the one on the other side of the trailer to compare. I am guessing your trailer has the type in the first link above, but you need to check to see. You will also need to measure the inside diameter of the hub where the cap goes to get the right size. Hope this helps.