When I've arrived at TBIT, I've been able to grab a nice big sturdy baggage cart for free in the pre-customs international bag claim area and keep it basically as long as I like. In particular, I've used the cart to go up the elevator to departures level, then outside of TBIT to walk to the DL check in area at T5, and then with my carryon put to the TSA security check line, where I was forced to abandon the cart.