I beleive these are the shoes you would need: http://www.napaonline.com/Catalog/Ca...154_0319322455
They have some that are more expensive so I'm not sure what would be sufficient. Since you have four tires instead of two I would guess there is less stress put on each shoe.
You also may be able to get your bearing parts at an auto store: http://www.napaonline.com/Catalog/Re...+2026026+25003
Not sure if the boat trailer versions are different/more durable?