Define "heavy duty" suspension. Cargo trailers are pretty much built for hauling the most common cargo, household goods, machines, tools, etc. Expected weight to be carried in such a small trailer doesn't warrant a "HD" suspension.
I suggest you just find a local cargo trailer manufacture and order what you want. Might consider something a bit bigger too, I wouldn't go smaller than a 6X10 to 6x14 Vnose. More useful and will hold their value.
A 5x8 will be limited, not only by weight but as to useable space. Doubt you find a 5X8 with standing headroom.
And, when it's time to sell that trailer I doubt you'd have much demand for it.
Which brings me to the least expensive way to get one, find a used one and change the axel yourself. Just know it won't be worth much more.