The iboats forum moderators are excited to announce that at this time it appears the iboats forums posting pictures function is working once again. Though we cannot promise for how long, we're hoping for indefinitely.
Look at the old one as there is a number on it. There is also a "sticky" at the very top of this page titled "Wheel Bearing Sizes". Be sure to check both the inner and outer bearings as they may or may not be the same size depending on the capacity of the trailer. If in doubt, take them with you to NAPA, Auto Zone, Pep Boys or any other major chain and they can get you the pieces.
Yah...they're getting rarer and rarer out there. The online trailer supply outfits are the best place to find them.
It's even harder finding new hubs for 3/4" axles. My current trailer, under a little aluminum boat, came with the wheels with built-in hubs. I had to replace them, and it took some searching to find the hubs.