Blinker fluid, like many ad hoc car maintenance and service products, is something you can find at most larger gas stations
Like, check near the automotive additives section, it's usually adjacent to items like radiator sealant or headlight restoration kits
In case it's not clearly labeled, make sure you check the bottom shelves
I don't know what country you're in, but I know that, in Germany and Austria, some stations keep it alongside seasonal products like winter wiper fluid
For the best results, at least in my experience, I recommend the synthetic variant rated for both incandescent and LED indicators