Fortunately, mixing home-brew formulas is fairly easy. You'll need a scale, but they aren't too expensive. For phenidone-based formulas, the scale should be capable of measuring pretty small quantities, or you'll need to make a stock solution in propylene glycol or some other suitable solvent. Of course, you'll also need a stock of raw chemicals. You can buy what you need from Photographer's Formulary, Art Craft, Digital Truth, or other outfits. Putting this all together will take a bit of research, but once you've got it you'll be able to experiment with lots of different formulas. Of course, buying ready-mixed developers is easier, but not by that much once you've got the necessary equipment and basic chemical stocks.