Smooth-on.com suppliers of material used to make things including prosthetic limbs. I think you make a plaster cast of the body part, then you can make that part of the body and mold it with changes, then take another plaster cast of the modified body part so you have two molds then join them and pour in gels to form the item in a particular silicone (safe for skin and body) then one side of the modified limb fits to your natural body surface and the modified limb is displayed. Then make-up can be applied to suit your skin color. The making of certain parts can be utilized to other areas like breasts, hips, buttocks but the hips and buttocks are subject to a lot of stress. Example sitting and standing and walking and also falling.
Also realbreasts obtained from thebreastformstore are made by realdoll and also see femskin too for female skinned like clothes, masks etc are worthwhile a look.
There is a lot of time taken in preparation, and the outcome really depends on your artistic and sculpting skills. One off casts cost a lot because this is the expertise in the make-up artist and the time they allocate to the job. So for a hobby at home the cost in time and wastage maybe high but if you are a creative person, heck why not try but make them medically safe too.
The important thing to realize is the gel is safe to use and not use any silicone due to vapors etc being absorbed into the skin and body contaminating it.