If you are looking an NKK Switches part number with an RO suffix ( i.e. the ordering number with the “-RO”), you may find the same number without “-RO” instead. For example, when searching for M2018B2B1W01-RO the part number M2018B2B1W01 will be listed.
Confirmed by NKK, the suffix “-RO” is no longer needed as all NKK Switches standard switches and accessories have been RoHS compliant since April 2006.
Here is the Environmental Compliance Declaration from supplier.