Electrical – Powering Arduino and motors on robot

What you would be looking for is a boost regulator, e.g. TI's LMR61428 or SemTech's SC4501.

An easy way to identify potential parts that meet your requirement is to search for "boost regulator" on a site like DigiKey, then apply filters in the parametric search for the characteristics you require.

Also useful is some exploration of online tools for power management design, such as TI's Simple Switcher tools. Each manufacturer who offers such a tool, would propose parts and designs built around their respective product ranges, so it would behoove you to use several such tools to understand the types and prices of parts involved, before making a decision.

This exploratory exercise will also help gain some understanding of the simplest ways to implement a boost regulator for your project, if needed. Many of the free design tools go all the way through to providing reference schematics based on your inputs, plus costing for the bill-of-materials involved, and PCB area budget.

As others have already mentioned, stepper motors draw power continuously when they are "ON". The data sheet says the winding resistance is 32.6 ohms, so applying Ohm's law you can determine how much current each winding will draw from the battery:

V = I x R --> I = V / R = 12 V / 32.6 ohms = 0.368 Amps

which is roughly the motor's rated current specification of 0.33A (as stated on SparkFun's website). Each bipolar stepper motor has two windings, each continuously drawing 0.368 amps; therefore, each motor continuously draws approximately 0.736 amps when it is energized.

Note that a stepper motor's shaft position is maintained only when the stepper motor is energized. If you de-energize the motor (turn it "OFF"), there is no magnetic field to maintain the shaft's current position, and the shaft free rotates.

Also, the Arduino's digital I/O pins cannot provide enough drive current to adequately energize your stepper motor's windings. Typically, the Arduino sends its low power digital "control" signals to an H-bridge circuit that provides the required voltage polarity and amperage to the stepper motor's windings.