Plastic Ballista by Eccentric Miniatures (partially built).
One of the great finds I came away from "Fall In!" with was a couple of packages of Eccentric Miniatures' "Great Crossbows". (Thanks, Bill, for pointing them out to me!)
These are plastic kits, sold 2 to a pack for $10 US.
I've started putting one together and have posed it with some of our Serqu figures. As you can see, it scales quite well with our stuff. Technically we don't have any artillery crew figures yet but I think the Serqu figures shown have possibilities. The figures are the three spear poses plus one of the archer poses. My idea is to leave off the plumes (and probably fill in the little dimple at the back of the helm). The plan with the archer figure is to cut away his bow and substitute a bolt from one of the sprues in the kit. He has a quiver of arrows - which is separate - though the straps are present. I think I am up to sculpting a replacement quiver using more of those loose bolts from the sprue.
I asked at the stand about the possibilities of catapults, and they said that a Trebuchet was being worked on. If they do that, then hopefully and onager or mangonel of some sort will follow. And I've mentioned siege engines to them as well - battering rams and towers, and the like. The kit goes together easily with very little fuss. I used GW's "thin" glue to fix the parts together. The bases are 50mm MDF from Gale Force Nine (bought "by the pint" at a previous convention.)
The scale is right. The price is right. What more can one ask for?