it’s true of you already have atx and you need to buy a sff psu. But there are a couple companies coming hard to the sff market. Cooler master has cheaper high power units out now and are said to be bringing kits. Case psu and AIO
How small are you looking to go? Give a range. < 10L? < 15L? < 20L?
The most popular bang for the buck sff case is the cm nr200 for $100. Is is on the larger side at 18L but being bigger means you won’t need specialized parts, cables etc. Good for entry into the SFF hobby.