As others have mentioned, the wiki lists locations that sell the detailed surface scanner.
I offer another, more generic solution, in the form of EDDB.
EDDB is the Swiss Army Knife to any commander.
For instance, to find the detailed surface scanner nearest to your location, click on the "Stations" tab.
In the field "Stations sells Modules" type "Det" and it should auto-complete to the detailed surface scanner.
Next, enter the system (not the station) where you find yourself into the "Reference System" box. Example: I am currently at Ohm station in LHS 20, so I enter "LHS 20" for my reference system.
Tip: It lists in between brackets the station names that are in that
system. Check your current station is in that list. If not, then you
may have mistyped your system. Some systems have very cryptic, coded
designations that closely match other systems and so can easily be
Finally click the "Find Stations" button and wait for the results to come back. The results are sorted on distance (shortest to longest) from your system and will include the station name where you need to dock or land. It does not list the price however.
Use the same website for finding other parts.
You can also use EDDB for trading and other purposes, but now we're veering OT. If interested in these other services, hit your favourite search engine.