So I have a little bit different take on this. I used to think I had flat feet, no arch, but in fact what was happening was that I was over-pronated, which also resulted in my being knock-kneed. In other words, my arch was flattened, which pushed my ankle bones inwards, and in turn forced my knee inwards also. If that is the OP's situation, then I think the best correction is to actually correct the over-pronation. The reason I say this is that if you are over-pronated and knock kneed, that really puts a strain on your knee joint. Your leg joints are designed so that the weight of your body goes in a straight line through the middle of your knee joint and the middle of your ankle joint. Or as one of my teachers put it, the knee is a weight transfer joint, not a weight bearing joint. If your knee, ankle and foot are not aligned, then you will be more likely to suffer degenerative arthritis in your joints in later years, as with every step you are effectively twisting/straining your knee and ankle joints.
I learned this the first time I was fitted for foot beds way back in the 80's. The first time the fitter aligned my knees so they were directly over my feet, I felt like I was standing bow-legged, but in fact this was correcting my chronic over-pronation. The lesson was reinforced by my training in tai chi, where the instructor was adamant that I have my knee over my foot. It took me a few years of retraining my feet and legs, but now when I stand I have some arch, and my body weight goes through the middle of my knee and ankle joints directly into the ground. One test that your joints are aligned is that when you stand with your feet together and bend your knees, your knees do not knock together, nor do they spread apart, but track parallel smoothly as you bend and straighten up.
So, if that is the case for the OP, then I suggest a custom foot bed to correct the overpronation. You can also practice correct knee and ankle alignment in street shoes, as I did, or get custom footbeds for those as well. Well worth it for the long term health of your leg joints