I'm not sure if you can go out and buy it like a loaf of bread. You'll probably need to contact your county or state's agricultural department. You also can't apply it yourself, you need a certified pesticide applicator to do it for you and/or be present.
https://extension.uga.edu/content/d...y/anr/ponds/sportfishbulletinwebpage.pdf"Rotenone can be used to eliminate unwanted fish to make a new start on balancing
the pond fish population. Rotenone is effective in ponds with deep water. Several
formulations containing rotenone have been developed that have different effective
rates that depend on the water depth, water temperature, and fish species. Rotenone
biodegrades in four days to five weeks, depending on water temperature. Avoid
applications where overflow of the treated pond may drain into other fish-bearing
water. Detoxify rotenone by applying an equal amount of potassium permanganate
or chlorine. Rotenone products must be used according to their label instructions
and are restricted use pesticides.
A certified pesticide applicator must be directly
involved in application of rotenone and appropriate permits and permissions must
be obtained." 
