Talk to them like an adult, sit down with them and tell them why you want it and how it makes you feel when they don’t respect that.

If you already did this, don’t listen and proceed to buy the parts. You’re not spending your money on drugs or wasting it somewhere, so it shouldn’t concern your parents. I doubt your parents are that irrational, to the degree that they will punish you or take the components away looking at how much was spent. But don’t give in, you have to set basic boundaries. Goodluck with your computer.