Replacing baseboard registers in 1950's house

The baseboard registers and returns in our house built in 1951 have layers of years of repainting on them. We would like to replace them OR strip them and repaint if that is feasible. I have searched for replacements and haven't found the 13.75"x6.5" size. Any suggestions?
q replacing baseboard registers in 1950 s house, home maintenance repairs, minor home repair
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