Turnbuckles should be available in the "Fasteners" section of stores like Home Depot. I know that at my local Home Depot stores they're in the same aisle as the screws, nails, bolts, etc.
Suggestions:
(1) Make sure you have a good anchor at the front of the truck, like a tab on your bumper or brush guard.
(2) Don't put 'em on till you start wheeling, for two reasons: (a) might be illegal due to hazard posed to pedestrians and (b) whistling sound at speed will drive you crazy!
Have fun making 'em, Melissa!

Take pictures and post 'em here for everyone to see!
Andy