I have checked Northern Tool, Home Depot, Lowes, Tractor supply, and all the other major chains but they only have ones that are 63" on the in bed sections which will not fit in our trucks. Weatherguard makes one but it is very expensive.
If you have a bed liner you need to make sure that the width of the portion that dips into the truck bed is less than or equal to 58", lips on either side can be like 5" ach, and heihgt about 9 and 3/8". Leonard sells a truck box specifically for T100. It is made by UWS. I don't remember the model number. It cost $240-280.