Where does a collector go to find out the value of DDO's that aren't listed in RedBook?

For instance, the 1887 IHC FS-009.5 DDO, how does one get substantiated values for this in grades of MS?
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Recipient of the coveted "You Suck" award, April 2009 for cherrypicking a 1833 CBHD LM-5, and April 2022 for a 1835 LM-12, and again in Aug 2012 for picking off a 1952 FS-902.
If you ask, I can update the info on any variety here.