Open the galaxy map, choose the View tab, go down to Trade Routes and click Clear, then select the particular commodity. E.g. if you're looking for Palladium, open up Metals and check the appropriate check box. This will show animated Palladium trade routes for your current system and for any systems for which you have trade data; if a system is exporting Palladium then it stands to reason there's a supply there.
If no trade routes are shown then there's no Palladium being traded among the systems for which you have trade data; you can buy further data from likely looking nearby systems, just be sure to buy data from systems with the appropriate economy type. You can identify economy types in the same part of the interface, at the top of the dialog box.