This is a list of onion services run by the Debian project. Most of them are served from several backends using OnionBalance.
10years.debconf.org: http://b5tearqs4v4nvbup.onion/
appstream.debian.org: http://5j7saze5byfqccf3.onion/
apt.buildd.debian.org: http://ito4xpoj3re4wctm.onion/
backports.debian.org: http://6f6ejaiiixypfqaf.onion/
bits.debian.org: http://4ypuji3wwrg5zoxm.onion/
blends.debian.org: http://bcwpy5wca456u7tz.onion/
bootstrap.debian.net: http://ihdhoeoovbtgutfm.onion/
cdimage-search.debian.org: http://4zhlmuhqvjkvspwb.onion/
d-i.debian.org: http://f6syxyjdgzbeacry.onion/
debaday.debian.net: http://ammd7ytxcpeavif2.onion/
debconf0.debconf.org: http://ynr7muu3263jikep.onion/
debconf1.debconf.org: http://4do6yq4iwstidagh.onion/
debconf16.debconf.org: http://6nhxqcogfcwqzgnm.onion/
debconf2.debconf.org: http://ugw3zjsayleoamaz.onion/
debconf3.debconf.org: http://zdfsyv3rubuhpql3.onion/
debconf4.debconf.org: http://eeblrw5eh2is36az.onion/
debconf5.debconf.org: http://3m2tlhjsoxws2akz.onion/
debconf6.debconf.org: http://gmi5gld3uk5ozvrv.onion/
debconf7.debconf.org: http://465rf3c2oskkqc24.onion/
debdeltas.debian.net: http://vral2uljb3ndhhxr.onion/
debug.mirrors.debian.org: http://ktqxbqrhg5ai2c7f.onion/
dpl.debian.org: http://j73wbfpplklpixbh.onion/
dsa.debian.org: http://f7bphdxlqca3sevt.onion/
es.debconf.org: http://nwvk3svshonwqfjs.onion/
fr.debconf.org: http://ythg247lqkx2gpgx.onion/
ftp.debian.org: http://vwakviie2ienjx6t.onion/
ftp.ports.debian.org: http://nbybwh4atabu6xq3.onion/
incoming.debian.org: http://oscbw3h7wrfxqi4m.onion/
incoming.ports.debian.org: http://vyrxto4jsgoxvilf.onion/
lintian.debian.org: http://ohusanrieoxsxlmh.onion/
manpages.debian.org: http://pugljpwjhbiagkrn.onion/
metadata.ftp-master.debian.org: http://cmgvqnxjoiqthvrc.onion/
micronews.debian.org: http://n7jzk5wpel4tdog2.onion/
miniconf10.debconf.org: http://tpez4zz5a4civ6ew.onion/
mirror-master.debian.org: http://kyk55bof3hzdiwrm.onion/
mozilla.debian.net: http://fkbjngvraoici6k7.onion/
news.debian.net: http://tz4732fxpkehod36.onion/
onion.debian.org: http://5nca3wxl33tzlzj5.onion/
people.debian.org: http://hd37oiauf5uoz7gg.onion/
planet.debian.org: http://gnvweaoe2xzjqldu.onion/
release.debian.org: http://6nvqpgx7bih375fx.onion/
rtc.debian.org: http://ex4gh7cig5ssn2xm.onion/
security-team.debian.org: http://ynvs3km32u33agwq.onion/
security.debian.org: http://sgvtcaew4bxjd7ln.onion/
timeline.debian.net: http://qqvyib4j3fz66nuc.onion/
tracker.debian.org: http://2qlvvvnhqyda2ahd.onion/
wnpp-by-tags.debian.net: http://gl3n4wtekbfaubye.onion/
www.debian.org: http://sejnfjrq6szgca7v.onion/
www.ports.debian.org: http://lljrzrimek6if67j.onion/
In particular, once you have the apt-transport-tor package installed, the following entries should work in your sources list for a stable system:
deb tor+http://vwakviie2ienjx6t.onion/debian jessie main
deb tor+http://vwakviie2ienjx6t.onion/debian jessie-updates main
deb tor+http://sgvtcaew4bxjd7ln.onion/debian-security jessie/updates main
#deb tor+http://vwakviie2ienjx6t.onion/debian jessie-backports main