Uncommon resource are available on all planets but are required for almost everything and so highly sought after, especially Thamium9.
Thamium9 is a Isotope Element used in many ways from recharging to crafting.
Used in every seed recipe when farming as well as in the creation of Warp Cells and many Weapons and Cooling upgrades it is something you will need insane quantities of.
Behind simply buying from traders on Trading platforms and Freighters, the best way I have found myself is this planet listed below.
Platinum is an Uncommon Silicate required in the production of Shielding Shards, Plates and Sheets, being a far more efficient way to boost your Exo-suit and Ship shields than using Raw Oxides.
Zinc is an Uncommon Oxide Element, while it's obvious use in recharging Exo-Suit and Ship shields, it is one of the primary ingredients to creating Anti-Matter for Warp Cells and required for many upgrades to you Suit and Ship than span multiple mechanic uses.
Marrow Bulbs are a powerful item gained from mining specific plants within the cave systems of all planets.
They can be identified very easily via the glow they emit.
Marrow Bulbs are used primarily in creating Voltaic Cells which are essentially space batteries used in many different Home Base related blueprints.
All Exo-Craft pads require 2 Voltaic Cells, Beacons Require 1 to, Landing Pads require 5, at 50 Marrow Bulbs per cell you can end up needing thousands.
Plutonium is a common Isotope used to recharge all Weapon systems and Pulse Drive as well as being required to craft many things.
Plutonium is just about everywhere but not in such quantities as those found on the planet listed below.
Where To Obtain
Here is a planet with the most prolific quantities I have seen as of yet of the Uncommon Elements needed for just about everything that only requires the first 2 Portal Glyphs to dial:
It is a planet with a Cold Biome and dense forests.
I have placed a Beacon next to a Galactic Trade Terminal quite close to the gate.
Low Sentinel activity.
Frostwort also grows on this planet as an added bonus.