There are two methods of finding Material Traders close to you. One way is to use Inara, the other is the in-game tool. The in-game tool is limited to traders in systems you have visited before, while Inara keeps a record of all known Material Traders.
Inara.cz Trader Search
To find the Mat Traders on Inara.cz, click on “Galaxy” > “Systems and Stations” > “Nearest” > “Material Trader”. You should see the traders nearest to your position (as long as you are signed in, and your location is in the search box).
Each trader is listed by type of material traded, station, and system. You’ll see the distance to the systems; choose the one you want, and go grab your materials.
Galaxy Map Trader Search
On the galaxy map, choose the Map Configuration View, select the option (#2) block, and choose services. You’ll see a menu below the options block to choose the type of trader you want. They are color-coded, so any traders you have visited will show up on your galaxy map with that color.