They have to be tight to work.
1) you don’t need a prescription to buy them (at least in the US). I do mail order for mine all the time.
2) The first time you buy them, go to a person that will fit you. They will tell you how much pressure (15–20, 20–30, 30–40, etc.) and whether you need knee high or thigh high. Don’t guess. The wrong guess can be damaging to your health.
3) Cheap is not always good.
4) If you wear them, make sure that you rub moisturizer or lotion over your legs every day or so, to keep your skin moist. Compression stockings can absorb moisture from your legs and cause them to dry and crack.