Although there's most likely more systems than this in the game that have every material, and other systems that have higher percentages of individual materials, I just thought I'd share this here for the sake of convenience, so you have a system you can go to if you don't know where to find a specific material. The planet/moon with the highest concentration of a material has bold text, and if you at some point need to restock SRVs, the outpost orbiting A5 has restock facilities, but no large landing pad.
Very common materials aren't included since all of them are always present on every planet, but if you need them, the planets with the highest concentration of them are: Carbon on A5A (23.5%), Iron on A1 (37.1%), Nickel on A1 (28.1%), Phosphorus on A5A (15.1%) and Sulphur on A5A (28.0%). A1 may have a higher concentration of Carbon, Phosphorus and Sulphur, but they show up as bugged at 0.0% on the system map.
Common
Chromium: A2 (9.5%), A3 (9.5%), A4A (8.7%), A5B (5.5%), A5C (5.6%), A5D (5.5%), A6A (5.5%), A6C (5.5%)
Germanium: A4A (5.6%), A5A (3.7%), A5B (3.5%), A6C (3.5%)
Manganese: A1 (15.3%), A2 (8.7%), A3 (8.8%), A6A (5.1%), A6B (5.1%), A6C (5.0%)
Vanadium: A1 (9.1%), A3 (5.2%), A5A (3.2%), A5D (3.0%)
Zinc: A2 (5.7%)
Standard
Arsenic: A5C (1.6%), A6A (1.6%), A6B (1.6%)
Niobium: A1 (2.5%), A3 (1.4%), A4A (1.3%), A5C (0.8%), A5D (0.8%), A6A (0.8%)
Selenium: A4A (2.9%), A5B (4.2%), A5C (4.2%), A5D (4.2%), A6B (4.3%)
Tungsten: A3 (1.2%), A5C (0.7%), A6A (0.7%)
Zirconium: A1 (4.3%), A5A (1.5%)
Rare
Cadmium: A5A (1.0%), A5B (0.9%), A6B (1.0%), A6C (0.9%)
Mercury: A2 (0.9%), A5D (0.5%), A6B (0.5%), A6C (0.5%)
Molybdenum: A1 (2.4%), A2 (1.4%), A5A (0.8%)
Tin: A4A (1.2%), A5B (0.7%)
Yttrium: A2 (1.3%), A3 (1.3%), A6A (0.7%)
Very rare
Antimony: A5C (0.8%)
Polonium: A1 (1.1%)
Ruthenium: A5D (0.8%)
Technetium: A5A (0.5%), A5B (0.4%), A6B (0.4%)
Tellurium: A4A (1.4%), A6C (0.9%)
In case anyone's wondering, A1 is a 1.2 G metal rich planet 9.3 Ls from the star. A2 and A3 are close binary HMCs at 1.1 and 1.0 G respectively, located 27 Ls from the star. The rest consists of sub-0.1 G rocky and icy moons orbiting three gas giants. A4 is 1200 Ls from the star, A5 (has outpost with restock) is 1800 Ls and A6 is 2300 Ls. All planets and moons have pristine resources, and the terrain is relatively flat on most of them.