Petp is an excellent candidate for irregular shapes with small details. Though, other polymers are good for that, and I feel the process is the limit, more than the material.
A mould for plastic injection costs $$$$$. Is your aim to avoid this cost? If not, just ask your favorite injector, he has all polymer granules at hand.
Polymers can be shaped by suction in a warm mould. This is reasonably cheap for small series. Then, a polymer film is the base material and is cheap.
Having paid an affordable mould (milled for instance), you can can produce the objects by electrodeposition of nickel, aka
electroforming. These can be sterilized and resist UV, water and many chemicals indefinitely. One example among many
http://www.electroformednickel.com/servicesVery small series are the usual job for them.