I suggest you search eBay for a stepper motor driver that can handle around 3.5A. Something like [www.ebay.co.uk] which has a 3.3A setting.
I'm guessing that you will be moving the sample holder at slow speeds, in which case a 12V supply would be sufficient. But go for 24V if you want the possibility of higher speeds. The static power dissipation of that motor is about 25W maximum. Allowing for the power increasing when the motor is moving, and losses in the stepper driver, a 100W power supply should be more than enough. You can get a 12V or 24V 100W supply on eBay as well, e.g. [www.ebay.co.uk].
I suspect that for that application, you don't need such a powerful stepper motor.
I'm guessing that you will be moving the sample holder at slow speeds, in which case a 12V supply would be sufficient. But go for 24V if you want the possibility of higher speeds. The static power dissipation of that motor is about 25W maximum. Allowing for the power increasing when the motor is moving, and losses in the stepper driver, a 100W power supply should be more than enough. You can get a 12V or 24V 100W supply on eBay as well, e.g. [www.ebay.co.uk].
I suspect that for that application, you don't need such a powerful stepper motor.