I assume we're talking about the clearance lamps here. Often, the same style is used on the front, back, and both sides, with the only difference being the colour of the lens - that's the case on my Boler
There is little chance that the lamps are custom Bigfoot
items, since no one making trailers would bother doing that for their tiny production volume. I found a nearly exact match for my Boler
(although a different brand) in most of the RV stores. The bulbs to fit them should therefore be common.
Next, are we talking about lamps not working, or just the bulbs in them? For the bulb, I would just go to an auto parts store with a dead one and start comparing. The complete lamps might only be found from RV, truck, or trailer parts suppliers.
Val, can you post a closeup photo of one (lamp and/or bulb)?