That depends on what you mean by rescue floppy. Do you mean one that will contain the boot loader only in case you mess up your mbr, or do you want one that will contain a full fledged mini linux distro?
If you're after a true linux disk, I have seen several that fit on a pair of floppy disks. Check out www.distrowatch.org.
On the other hand, I would recommend a live CD, such as Knoppix.