Whatever you do, use a product labeled for spa use. Even though you won't get arrested for using pool chlorine, you are much better off using stuff labeled for spas. The correct amounts to add are given on spa products. Pool products will tell you what to add in 10,000 (or there about) gallon increments.
I always use my spa dealer. Yes, they are a bit more expensive, but they give me the service I want and sometimes need. Good luck getting that at Home Depot.