If you're trying to sell a specific commodity, the "exported to" system list in the commodities screen of the station services is a good start, but here's what I do to locate better info:
- Figure out what economies consume and produce the resource I want to sell. Either via the description,the ED wikia commodities info pages, or this handy spreadsheet
- Go to Galaxy Map
- Go to the 'view' options.
- Set "View by color" to "Economy"
- Un-set the economies that don't consume or produce that commodity (so it's easier to see the ones that do).
- Enable the "show trade routes" option.
- Clear all the trade routes (so nothing is shown)
- Enable the one commodity you're interested in.
- Buy up some trade data from systems that might need or produce that commodity. Not all systems have trade data available (especially further away), so including producers will increase the odds of finding a trade route for that.
- Look at the trade routes for that commodity. They're the brightly colored curved line of the same color as in the trade routes selection, with bright "sparks" traveling along them to show the direction the commodity flows. Make sure to watch for smaller sparks traveling backwards along a route. You want a system that the commodity flows into but doesn't flow out of. If there's a lot of that commodity flowing into a system, it might mean they have very high demand, but it may also mean their market is flooded with it and demand is now low.
- Now you should have some candidates. Click on them and go into the system view.
- When you're looking at the system, check all of the stations. Look at both the primary and secondary economy of each station to verify that your commodity isn't produced anywhere in that system. Verify that your commodity isn't illegal at that station. See if it's in the list of top 3 imports (or at least similar to one that is), and verify that it isn't an export. Try to verify that the station you want to travel to isn't too distant from the primary star.
- Switch to Navigation tab of the galaxy map, pick "Fastest" route option, click the "route to this system" button for the destination (if you intend to grab more cargo, make sure to slide that slider over).
- Optional: Do a quick check on the intermediate route systems to make sure my commodity isn't illegal in any of them. I think for that, you only need to check the station closest to the major star, since it will be the one whose faction sets the law for the space around the star that you'll be jumping through. If you're in a poorly armed ship carrying expensive cargo, you may also want to avoid anarchy systems. This is optional, since you can usually jump through intermediate route systems without spending enough time there to get interdicted by authority or pirate ships.
The same basic technique works well for searching for where to buy a specific commodity, such as when you have a mission.
In the example of H.E. Suits, those are imported by pretty much every economy except agriculture (maybe also not service and tourist?), and they're produced in high-tech systems.
Alternately, you can skip all of that, go to Thrudd's Elite Dangerous Trading Tool, an do Search/Commodity/Station Buying and put in your commodity, station, etc and see what price different stations were buying it for in the last couple of days.
Other trade tools: