There are different repositories for different versions, but if you need a REALLY old version then you'd have to find it elsewhere.
There are servers that store the programs, they have 95% of the software out there for Linux, and when you apt-get something, it downloads it from one of the many repositories. Debian.org serves one, but I use mirrors.kernel.org.
Hopefully that bash documentation helps, its a good read.
Linear Mode
