Electronic – Where to find air core inductive coils

A metal core is advisable because it concentrates the magnetic field. This makes magnetic coupling between coils more effective, but you can do it without the core too, especially if both coils are tightly coupled (mechanically close to each other). If you don't have a core and the two coils are some distance apart, part of the generated field will pass outside of the other coil.
(In a transformer the outside metal "shortcuts" the field, increasing it. Without this outside metal the field would fill a large volume around the core.)

I have to tackle similar problems on rotating machines - sending power and receiving data. Do you use power resonant coils? That's the first question. What sort of power transmission frequency do you use? Steel doesn't have to act like a shorted turn and if it does, it won't completely kill the mag field - with resonance you can re-tune and use the metal as part of the tuned circuit.

Trying to cancel parts of the magnetic field is a fruitless task - Sometimes I use ferrites when the metal (such as stainless with poor conductivity) is heating up too much. Normal steel is better but it still has losses and ferrite does the job of masking areas that would otherwise drain all the energy.

Some diagrams/pictures would help but the above are my thoughts to start off with.