Ideally you’d look for the specialty coffee roasters in your area/country and order freshly roasted coffee from them if the price suits you.
My personal “cheap” go to is from the Lidl brand (Bellaron?). It goes for about 8-10€/kilo, it’s a medium roast (lowest "intensity’) and tastes decent. Whole beans of course. It’s 100% arabica which is what you should go for anyway. Supermarket coffees are usually darker roasts and you might want to grind coarser than usual. Just don’t be discouraged if the first few brews aren’t to your liking. You might have to adjust. Good luck!