I think anyone can learn to use a sewing machine. Just like anyone can learn to do math :) It only takes a bit of practice and memorization ^_^
I know when I first started out sewing, everything I made either fell apart, or looked horrible! However now I plan to make one of my outfits for an upcoming Lolita fashion show!
If you find the right tutorials it can be really easy! It usually does involve a small bit of math(which is why I referenced it before) but not too much, just small simple calculations that you could probably use a calculator for if you needed to!
The fabric is really all you should worry about after that. Sometimes some light cotton fabric is just the right thing, others you may want some silky type fabric(which will be a bit more expensive) it all depends on the style you are looking for :)
If you have your heart set on buying clothes, not making them, try etsy shops, e-bay stores, or maybe commission someone here on MAF?