Missing data in particular data fields is a common and difficult problem. Depending on which data is missing, there are several resources that might help.
For finding individual missing ISBNs, there are WorldCat, OCLC and other large catalog resources. One can look at the physical book. A workaround is to use the look-up function in an OpenURL linker. The OpenURL linker will compare titles to the Knowledge Based and return the ISBNs. The tool used in these examples can look up the following fields:
Local (if the library used local numbers for resources in their OpenURL linker)
LCCN Control ...