I have mostly Late Roman Gripping Beast artillery pieces, which I love. However Foundry has the only light cheiroballistra that I've been able to find. I put them on wagons in my army
Also, if you would like the grand-daddy of all ballistae, Black Tree Design makes a 60-pounder and their annual Black Friday 50% off sale is next month. The BTD crew is EIR, I bought an extra Late Roman crew for mine.
They also have EIR ballistae which are pretty inexpensive if you get them on sale as well, I have a couple for
Broken Legions.