I have an older car with an unusual lug configuration so it is very difficult to get rims for it. Is it possible to get custom aluminum rims cast and machined to a specific lug configuration? If so, what search term should I use? I can't use "custom rim" because that just brings up regular production wheels.
Most aftermarket wheels are available in a custom PCD (lug configuration as you call it) as they are drilled to order to save having loads of stock.
Just ring and speak to some of the popular wheel brands and ask them if they will machine and supply a set in your desired PCD.
Another possibility is getting a used set re-dilled/machined to your PCD, depending on how different they are it could be very cost effective to do. It is actually better if the two PCDs are vastly different as it means there is more material in between the existing holes and the new ones, if they are too close it will require tig welding the old holes up. A good wheel specialist or machine shop should be able to do this.
Yes. The term you need to search on is "made-to-order".