tire and rim problem with Ford 445

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a rear tractor tire on my 1979 Ford 445 tractor has gone flat three times in the last few months. Seems that the rim may be the problem. Had cracks in the rim welded and rim polished smooth so as not to have any rough edges to puncture the tube. However, still got another flat! I am told I need a new rim, however have not been able to find a used OR a new rim to fit tire that is 169-24 on a 15" rim with 6 bolts. Anyone know where to find such a rim?