It's really up to you what you use. Store bought syrup would work, you'd have to adjust to your tastes. We personally are looking to reduce high fructose corn syrup in our food and drinks, so I make my syrups from scratch with cane sugar.
The recipe is usually simple: cocoa, sugar, water and some vanilla. You make a simple syrup (1-2 parts sugar to 1 part water, ounces work best) first and add the other stuff.
I used the recipe here
and it flavors the soda well at 1-1.5 oz per 8-12 oz glass. The only critique of this recipe is how thin it is in body. Flavor is good though, and it mixes easily.
For a great treat, add a splash of cream or a couple of scoops of ice cream!