LHS 3447 has extraction sites in the ring around LHS 3447 B3. The outpost "Lawson Orbital" orbits it.
Kremainn has extraction sites in the rings around Kremainn 2, a gas giant orbited by the coriolis station "Wohler Terminal"
Kremainn 4 is also a gas giant with rings and orbited by the outpost "Volta station", which is an industrial/refinery operation. Sometimes, they have their own extraction sites, too. Worth a look.
"Boswell Platform" and "MacArthur Terminal" in the LTT 18486 system both orbit gas giants with rings, and both are housing refineries. Chances are that you will find extraction sites in the planetary rings there as well. Both stations are outpost.
You have to approach the planet and get as near as 1000 ls minimum to see extraction sites pop up in your navigation tab. Simply jumping to a system does not reveal whether you can find any in there.