If you have the technical acumen, you might consider hacking the seeed motor shield. It uses a L298 dual H-bridge driver that is specified to run with motor supply voltages up to 40 VDC.
I can't find any schematics for the seeed motor shield, and I don't care to reverse engineer their Eagle files. Nor is there any explanation of why seeed specifies a maximum of 15 VDC. There is a two-pin connector on the shield, identified as "Supply Power Connector," with a jumper across it, identified as "MB_EN.," This was apparently intended to allow an external motor power supply, independent of the lower voltage USB power that is normally applied to the board. Seeed has this to say about that: The purpose of "Supply Power Connector" is using the jumper cap to select USB or external power for driving the motor, But you must insert the jumper cap now. We will later be resolved the problem. Yeah, sure.
I think seeed screwed up their design by not providing for the use of these two pins to provide motor power. It probably has something to do with the voltage limitations of the on-board 5V regulator chip. This should have been three pins, not two! One pin to supply power to the L298 for motor drive; one pin to connect to the external (USB) power; and the third pin to connect to an external motor power supply.
Seems to me that it should be possible to find the motor power Vc terminal on the L298 (check datasheet for pinouts corresponding to whatever physical package seeed used), isolate that pin from whatever else it is connected to, and provide that terminal with up to 40 VDC with respect to "ground" to drive motors with voltage requirements greater than 15 VDC but less than 40 VDC.
Don't take my word for it that this hack would be safe and effective for your 24 VDC stepper. Proceed at your own risk. I am just passing on what it says on the L298 datasheet.. I don't have a seeed motor shield, so I have no way to test the hack.
Hop