I used to do something similar with yum on CentOS. I think there was a --enable-repo and --disable-repo param.
Is there something that exists for
apt-get install <some pkg> that will restrict to
You can do that by pinning the release using
Package: * Pin: release o=Debian,a=testing Pin-Priority: 900
I don't believe you can do that quickly and easily without a wrapper around apt-get to juggle the sources manually.
One option however is to suffix the package you wish to install with the version number required, which will likely be different in each source.