Try an extract like McCormick, you should be able to find it at the grocery store, and it's not too far off of something similar to A&W, though you may want to add more vanilla with it.
Sprecher extract is a syrup and it contains preservatives, which is why you can't carbonate it naturally. Though if that's the route you're going, be careful with those glass bottles. You'll want a test bottle that PET so you can see how firm it's getting as it ferments.
If you want to avoid fermenting altogether, use the extract to make up a syrup and bottle that with some ice cold club soda.