You should be able to find at least a silver weapon (which will work on ghosts) in the Market District - try Slash & Smash, or the Three Brothers Trade Goods. Buying a magic item wouldn't really be worth the money.
You could also use spells on ghosts (like the spell "Flare" that you started the game with.)
[ *followup* ]
You don't buy flares - you already have the spell that is called Flare, that you can cast multiple times to kill a ghost.
In order to get 5 ectoplasms, you'll need to kill 5 ghosts, and loot their corpses.
It can also be found randomly elsewhere..
Guaranteed samples can be found in the following locations:
1 sample is in All Things Alchemical in Skingrad.
1 sample is at Meridia's Shrine.
It can also be found randomly in the inventories of alchemy vendors (starting at level 4) and in some Necromancer and Vampire loot chests (also starting at level 4).