The 2.25x16 Cheng Shin rib tires are no longer available. Cheng Shin stopped importing D.O.T. tires about 10 years ago.
You might find some old 2.25x16 uni tread Cheng Shin on Ebay. Which will be at least 10-15 years old, and might be too light of a tire if the weight across your front axle is over 350 lbs. There are a couple of 3.00x16 tires available, [$60 to $100 ea] but if you plan on running your slingshot for awhile, you may want to convert your wheels to 17" so you can get replacement tires.
Feel free to call me with any questions, 805-444-4489