I would try a local to you trailer shop. Get some dimensions from your good fender and take that with you (if you can't bring the trailer to them).
You could probably order something online from places like etrailer.com, but I'd be willing to bet that the local trailer shop can get you something really close to your original without a lot of guess work.
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1999 180 Horizon OB Fisherman Pkg
San Antonio, TX
Choice of tow vehicle:
'93 Explorer 4x4 (sold)
'05 Grand Cherokee 4x4 w/ 5.7 Hemi (member of the 240k+ mile club!)