Mouser sells both DS12887 modules and DS12885 ICs (the same IC as used inside DS12887, but without battery and crystal). They are not very cheap though. I bet there are many other places where you can buy them, but I like Mouser because they sell new components (other places might sell you modules made 10 years ago, with almost completely discharged batteries).
Last week I repaired two DS1287 modules in my Intel Professional Workstation boxes, and soldered / glued nice CR2032 battery holders on top of them. The process is really very simple. If needed I can post some pictures.
BTW Intel Professional Workstation is a really interesting box. Kind of 1990's Intel vision of PC of the future. It is 486 based, EISA bus, low profile (7.6 cm / 3" high) box, with on-board SCSI, Ethernet, VGA (all this in '91 or so). The box could be opened by unscrewing only one single screw, there is no wires inside the box except short IDE and HDD power cables (5 cm / 2" long or so). If people are interested, I can start another thread with system description...