Well, you don't need a lot of artistic ability to do a few simple symbolic drawings. The trick on a lot of drawing is to make it much larger than you're actually going to use it, as shrinking makes a lot of little flaws simply vanish. It also depends on what you are looking for: do you want each item to be unique, so Castle Vaardarasha has a different symbol from Castle Dundonerksh? You'll likely need to draw your own. You want to use a font for the symbols? There are a few that exist that'd do that. Brushes? What?
It might be helpful to at least let people know what software you are using to make your maps.