There are about 200 brew your own tea options if that's the route you want to go, but I'll need some more specific information.  There are a number of sugar-free (artificially sweetened) single serving powdered tea options, but none that are specifically labeled unsweetened.  Here's a list if you're interested.  The Arizona Tea options seem to be the most economical.

Cases of 90 individual powdered drink packets, from Arizon Tea, for 3.5 BTC:

• Arnold Palmer Half Iced Tea/Half Lemonade (not sugar free)
• Sugar Free Green Tea with Ginseng
• Sugar Free Lemon Iced Tea
• Sugar Free Peach Iced Tea
• Sugar Free Pomegranate Green Tea

Cases of 120 individual powdered drink packets, from Crystal Light, for 6 BTC:

• On the Go Iced Tea (sugar free)
• Peach Tea Drink

Case of 30 individual powdered drink packets (artificially sweetened, naturally and artificially flavored), from "Office Snax", for 2.1 BTC.