Want a recommendation for a serial EEPROM or other non-volatile memory
    in the 256k to 4Mb range to which a 12C509 may speak.

    Constraints:
            - eight pin dip,
            - 5v supply,
            - small quantities (10-50).

    For an antique computer, an 80C85 based Tandy 100/102/200 laptop, I have
    volunteered to a group of users on the 'net to investigate the
    construction of a simple device to act as offline storage.

Man, that's scary.  In two chips you can implement more storage than the
old 5.25 inch floppy drives.  (maybe you can make a commodore 64 or
atari version as well.  IIRC the floppies for those systems used an
rs232-like port.)

I don't think you can find serial EEPROM or flash that dense in that
sized package - the chips themselves get too big, do you get 28 pin TSOP
style packages with only a few pins connected (they should fit into an
rs232 clamshell OK, though.)  Check into the atmel "serial dataflash"
parts.  If you use a crystal on a slightly larger PIC (16C505?), you
shouldn't need a separate baud rate generator.

BillW