First of all: the 2.4 sources are of no use at all (for you). If you need to compile a driver, you'll need the sources that match your kernel exactly. I feel sorry for you that the 2.6 kernel is a little bigger, but there is no way around!
If you have 9.1 Pro, then the sources are on your installation medium. If you have 9.1 Personal, you need to download them from the server. Try to find a near mirror of the SuSE FTP server an add this to your installation sources in yast. Then you can use software install to install the kernel sources (search for the keyword 'source').
EDIT: OK, forget the last suggestion! Your modem does not run, right?
I guess you have intyernet access from a windows machine? Download the following file from the FTP server: kernel-source-2.6.4-52.i586.rpm
The path should be something like ftp.suse.com/suse/i386/9.1/suse/i586
You'll also need the rpm packages of make, gcc, glibc and glibc-devel (it's much easier if you buy a Pro version
)