no specific answers, but:
figure out about what you need (if you only need to turn it on/off, probably a gear-motor will work, just figure out about how much torque you need and similar and go looking around for something affordable...).
torque needed to open/close the valve will probably matter more than RPM, as two motors of the same speed could have very different torques (say, one is incapable of turning the valve, and another could damage or destroy the valve if it turns too far).
so, maybe turn the valve using a torque wrench and get a general range of values.
depending on specifics, a servomotor or stepper could also be more appropriate the task. these could better allow knowing when the valve has been turned, a servo by knowing how far it has turned, and a stepper may know how far it has tried to move (may not be accurate if the motor is overloaded or stalled, but is the step count is otherwise normally accurate).
servos normally already come reduced (so they are like gearmotors in this sense), and steppers are normally slow but often very strong (if big enough for the task).
I often look for stuff on Amazon as they tend to have things available for reasonable prices, though information about products is often a bit, lacking, and sellers may often take a long time to ship (or throw random parts together in paper or plastic bags and ship them without any other protection, leaving it to luck if they arrive undamaged...).
a lot of sites specifically for motors may have a wide selection and detailed stats, but tend to have high prices. sometimes, one can also find good options on Alibaba or AliExpress or similar (one can often get things from closer to the source, from China, or at least closer, like for parts made in places like Thailand or Vietnam or similar...).
actual selection is harder. one option is to lookup whatever info one can find (sometimes involves trying to decipher websites and datasheets written in Chinese and similar), and see if it was good (approximately matches requirements), and if there are no obvious cheaper options.
but, yeah, dunno, just a few quick thoughts...
