A good custom knife is by definition a high end knife and not cheap. Thus, the used material must match the expectations.
Normally, I build my kitchen knives with stainless steels. Therefore, all other used material should be corrosion resistant. For the handles, I am using stabilized wood or stock from bone or horn.
For pocket knifes, the emphasis is also laying on corrosion resistance. I am using here highend Titanium (grade 5 or 6), stainless steel and Bronze. The handles can be from Mammoth-Ivory, wood, Micata or G-10.
Popular is also giraffe bone, fossil sea-cow rib or Mammoth-teeth.