You can get the source code here, not the rpm or src.rpm, but the source code itself.
ftp://ftp.gnu.org/gnu/bash/
Here is an example on how to build it
2.3.10. Build BASH
Get the bash-2.05 source code package from
ftp://ftp.gnu.org/gnu/bash/ and untar it into the /usr/src directory.
Note
BASH version 2.05b, the latest version at the time of this writing, will not build successfully when using the --enable-minimal-config option. This leaves two choices. We can either fix 2.05b by applying the patch posted on gnu.bash.bug under the subject, "Compile error in execute_cmd.c with --enable-minimal-config" or we can simply use the 2.05a version.
bash# cd /usr/src/bash-2.05a
bash# ./configure --enable-static-link \
--enable-minimal-config --host=i386-pc-linux-gnu
bash# make
bash# strip bash
I found that here, I think they are building boot floppys from scratch
http://www.faqs.org/docs/pocket_linux/x167.html
With any GNU program, like bash, you can always get the source, somewhere. A src.RPM is just a system to package the source code. The RPM is just a system to package the already built binary. Hope that makes sense.
David