Well, you can got to the map panel first.
The get to the 4th tab from your left (on my ps4 it has stars icon)
Then from there you'll see "Map configuration"
There's a spot where it drops down a sub menu.
Select services.
From there you'll see a list of services.
On my ps4 the top 3 are the types of Material traders to choose from.
Select the one you're looking for and start searching the stars. They're far, few and in between. A real serious pain in the...... Well you get the idea.
Or you can go do what everyone else does (ain't I a bit annoying

) and look for the closest one to you at the below link.
Just put the name of the system you're in in the search window. That's all you need to do.
The list will show up. You'll see a few columns. The left is the trader type. The rest you can easily figure out.
I know I posted a link to The EDDB site for you in another post. But this one does some things that one won't, so it's another good reference tool to keep hold of.
The community site for Elite: Dangerous, Starfield, Kingdom Come: Deliverance with the game databases, information, gaming news and other handy content...
inara.cz