Just get some rectangular tubing and make them. If you want the car tied together even better, instead of just hanging them under the floor pan, cut a notch in the floor and let the ties protrude about 1" above the floorboard, then weld them to the floor, too. Also helps to run a brace the same size as the frame ties over to the point on the floor where the rollbar will weld in. Weld this brace with a plate on it to the inside of the rocker. As for size on the tubing, 2X2 or 1 1/2 X 3 inches works well, doesn't have to be a real thick wall, 11 ga. would do fine.