Check out the drug stores in your area. Some of them around here, at least carry even store brands of carbonated fruit flavored "water" sweetened with Spenda. Some are really good. I found 1 ltr bottles 3/$2. When visiting CT, I found them 4/$2 at a different store. Around here, only one of the grocery store chains carries diet rite. Some carry only cola, some carry other flavors, so you might try shopping a different part of town and see if the choices are different. Speaking (nicely!) to the managers of stores sometimes helps them decide to order more of the choices, too!!
I had given up around here, and then when buying school supplies in Walgreens, I found the fruit flavored carbonated stuff... It's really good.