You can use another
sources.list, and as muru pointed out, it’s as simple as
apt -o "Dir::Etc::sourcelist=/path/to/your/sources.list" source dos2unix
The documentation suggests that this isn’t possible except in a configuration file, but it turns out the documentation is wrong (see the revision history for the configuration file variant).
Alternatively, you could clone the package source directly, if the package is maintained in a revision control system.
apt showsrc dos2unix shows
so if you have
git installed you can clone that.
debcheckout, in the
devscripts package, can automate that for you, but you probably don’t have that installed... See How to know the source repository of a package in debian? for details.