Equipment searching can be tedious in Elite regardless of what equipment you are looking for.
There is no specific trick to finding a Detailed Surface Scanner, just generic advice for whenever you are searching for a particular piece of equipment.
Better equipment choice can be found in High Tech or Industrial systems. On the galaxy map, you can set a filter type to only show those sorts of planets. Wealthy systems with higher populations seem to give the widest choice - you can also set a filter to remove lower population systems.
When you've found a system that looks like a good candidate, look at the System Map and click on each of its stations. Check that they have Outfitting and also that their economies are High Tech or Industrial as it is possible to have a station with an Agricultural economy mixed in to a system with mostly High Tech economies (it seems to be based more around the planet that the station is orbiting).
Even after doing this, the stock at the stations is randomised and will change over time, so they might have one the first time you check, but it not be there when you have saved up enough to buy one.
If you are anywhere near Xihe, check that place out. It is my first stop shop for equipment - it always seemed to have everything you need to fully upgrade the smaller ships up to a Cobra Mk 3.
The galaxy simulation affects all players, regardless of game mode they play. If you don't want to interact with anybody, you can play in solo mode, and if you want to interact with just a group of friends you can play in a private group.
You cannot join a power and then have no competition... the other player's efforts will still affect yours.
Best Answer
The Elite: Dangerous galaxy is quite large, even if you only look at the areas populated by humans. If you're flying in areas that aren't popular, you could easily be in areas that haven't seen players in a long time. Your timezone and time of day could also effect it greatly.
I assume that you're already playing in Open, and know to look for hollow blips on your scanner.
Things you can do to increase your chances of running into other players: