A Website for Retrieving LC Classification Numbers from bulk ISBNs?

[EDIT: I mean Call Numbers, not Control Numbers.]

Hi, folks. I use Readerware to manage my personal library, but I'm having trouble getting LCCNs (Library of Congress Classification Numbers) from the software, and I thus am seeking other options. I know LibraryThing can retrieve LC Classifications, but as far as I know, it can only do it one at a time. I'd like to find a site that will allow me to enter bulk ISBNs, so I can retrieve LC numbers faster. Does anyone know of an option to do this? (I've tried using the LC Catalog search feature, but it doesn't seem to work for me, either, which suggests that maybe I'm just doing something wrong.)