Digikey has a category of products for this, dc-dc converters:
For example, this would work but it costs around 30$ :
https://www.digikey.com/product-detail/en/tdk-lambda-americas-inc/I6A24014A033V-001-R/285-2434-ND/5481911The datasheet only shows 14v in -> 12v out in graphs, but it will probably work with as little as 13v.
If you want super cheap and don't care about efficiency, you could just grab two or three LM1084 (linear regulators) and parallel them using a 0.1 ohm resistor for each one as balancing resistors. See page 15 in
https://www.analog.com/media/en/technical-documentation/data-sheets/108345fh.pdf but replace "2 feet of AWG18 wire" with a 0.05 ohm 1w resistor or something like that (AWG18 has around 0.021 ohm per meter/3ft of resistance)
Each 1084 has a peak current of 5A, so if you use 3, each regulator would have around 3.3A of current going through it.
The 1084 has a typical voltage drop of around 1.3A at 5A of current, but keep in mind they must be cooled... the total amount of heat they can dissipate is around 15 watts.
So LM1084/LT1084/*1084 could be used to output 12v with as little as 13.5v
Also, you should think about the maximum voltage the motor supports.
When the voltage drops below some threshold, it may be easier and cheaper for you to just use a mechanical relay to switch a dc-dc regulator and direct / through a bridge rectifier
Also... if you know the voltage is not gonna be above 14-15v and your motor isn't super picky about voltage then in theory you could use a bridge rectifier and a mechanical relay to drop some voltage
A bridge rectifier has 4 diodes inside, and depending on how you connect the wires, you could have two diodes in series to drop around 2v or you could have one diode and drop only 1v
For example see this 1.25$ 20A bridge rectifier :
https://www.digikey.com/product-detail/en/smc-diode-solutions/GBJ2006TB/1655-1863-ND/7244891
Could have a voltage comparator or microcontroller or something enable the relay only when voltage goes above 13v... in default state relay is connected so that only 1v is dropped, if voltage is above 13v relay turns on and switching changes so you drop 2 x 1-1.1v
A 12v relay would work with 10v..15v, no worries.