Disclaimer: not an expert. I have made gum arabic from the the spray dried powder--you do indeed need a preservative or like any food based product the gum will rot quickly. Two options I looked into are thymol and sodium benzoate.

Thymol is used in antiseptics. It will not dissolve in water, but must be used in alcohol. My notes say to use 10 grams in 10ml of alcohol. I'm sorry to say I haven't used thymol personally so I don't know if that's isopropyl alcohol from drugstores, or grain alcohol like everclear; nor do I know how much gum solution this goes into.

Sodium benzoate is a food preservative and you can find it on the label of soft drinks. It readily dissolves in water. I use this in my gum solutions and it works well. You can get it from the Photographer's Formulary. I use 0.5 grams for every 100 grams of dry gum arabic powder. The standard ratio for making gum arabic is one part gum powder to two parts water.

For what it's worth, the stuff I make from scratch looks (same dark tan color) and acts exactly like the premade gum arabic solution sold from Bostick and Sullivan. It's definitely fun to make your own, but in terms of cost and convenience--making it versus buying a premade bottle of gum--my opinion is that one isn't any better than the other. Good luck!