Spell Helms

A most rare device of unknown origin. Lost to time. But now unburied when The World Spine grew and now dominates the land. With the proper modifications this can turn nearly any vehicle into a space capable ship. But it needs a lot of power put into it to thrust you into the sky. Think of it as a horse that pulls a cart, those guys eat a lot of food. Poop too. But that's taken care of by the city so just don't step in it.
It needs to be energized by somebody capable of using and controlling arcane power. Wizards tend to fit the bill best but it drains them of their resources so they tend to complain about that. Rogues will often have enough bravado that they can power them through pure strength of personality. Got a problem rogue with too much time on his hands? Put him on the deck and have him run the ship. No more idle hands. No more doing the work of Orcus with them.
The magic to create new ones is unknown even to the gods. There used to be scrolls that could create them but those are so valuable that they tend to be sold in the mega-cities out in space. So low chance of acquiring one on Lloegyr. No, it's best to find an intact enough old one that still works. Or maybe works some of the time. What's the worst that can happen? You are unlikely to crash into anything in space so you just keep heading where you were heading with nothing to stop you. And since you were heading there, isn't that what you wanted anyways? It seems to the author to be a self-correcting problem. But we digress.
Okay right, it's best to find one that is still intact. Remove from that ship and install it into yours. They are still quite bulky and extremely heavy so bring your best hauling elephants, roller, block-and-tackles, and what have you. Now you can see why the scrolls all get sold off. They're so light and easy to transport, might as well sell it to the guy who controls a galaxy whose sun shits out diamonds. There's only so many diamonds you can reasonably wear before it gets gaudy. I kid. No there isn't. But these are the types who have enough to share and to make sure their more far-reaching holdings have enough ships to operate properly.