Shimano brakes usually come with them as the majority of bicycles are IS mount.
You have to be careful as some of the cheapest Shimano hydraulic are usually OEM packaged, meaning the housing could be a touch too short, no IS to post mount etc.
Other option is Clark M2/Clout (they're £30 on Amazon with free shipping).
Does that mean I can sell the current adapters with the Spyre calipers when the time comes?
Yup, some adaptor are usually specific to brakes, as mad as it sound, not all calipers fit on all adaptor (despite the fact they're literally just put the brake in the position for the rotor size.
Shimano brakes usually come with them as the majority of bicycles are IS mount.
You have to be careful as some of the cheapest Shimano hydraulic are usually OEM packaged, meaning the housing could be a touch too short, no IS to post mount etc.
Other option is Clark M2/Clout (they're £30 on Amazon with free shipping).