All files are signed with my Qubes OS Signing key:
pub 4096R/C8C0C2EF 2016-06-25
Key fingerprint = 4B1F 400D F256 51B5 3C41 41B3 8B3F 30F9 C8C0 C2EF
uid unman (Qubes OS signing key)
sub 4096R/4731B36C 2016-06-27 [expires: 2022-06-30]
You'll need to get this from a keyserver, or two, to make sure all is fine.
You can also check the Qubes users mailing list or github.
To use a Ubuntu repository, download the key, copy it to a Ubuntu Template, and install it using apt-key.
Then add a line like this to /etc/apt/sources.list:
deb https://qubes.3isec.org/3.2 xenial main
deb https://qubes.3isec.org/4.0 bionic main
Replace 'bionic' with the code name of your version.