Find class 3 gimballed multi-cannon

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.

You can't. Currently, the class ranking of weapons doesn't seem to mean anything at all. Your choices of weapons are strictly limited by type, size, and targeting method. There aren't higher class small fixed pulse lasers, for example, there is only one variety of small fixed pulse laser.

What I think is going on here, is the game is ranking weapons based on their overall stats, regardless of power cost, size, or targeting method. For example, a huge plasma accelerator is class A, a medium railgun is class B, and a small fixed pulse laser is class F. There's not a class D small fixed pulse laser, because that's not how that class stat works. You can check out all possible weapons that you can find in the Elite Shipyard.