I gave up on 13" tires on my boat trailer. I never found a "D" rated tire either and "C" rated tires were hard to find. The last ones I did find were 18 months old and still sitting on the shelf. Fortunately my boat trailer comes from the factory with 13's or 14's so I just replaced all 4 wheels with 14's and got the tires I wanted. I originally bought this trailer with 13's because I needed the trailer to be as low as possible so I could fit it into a garage. With a new garage with 8' clear doors that isn't a problem now.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50C, TST Systems 507 TPMS, RV Flex Armor Roof