I did a DIY FMT and used a parent as a donor as he/she was the healthiest in my family. I have 1-2 cousins I could potentially ask, based on their health and diet, but think even that would be took big and weird of an ask.
If you go to a clinic or a doctor's, they do the quality control for you. You'd have to ask them what their criteria is, but I wouldn't trust it that much. There is a case study of a C.Diff patient who received a FMT from an obese donor and subsequently became obese because of the donor microflora.