Implant casting is one way to avoid the rough surface left by carving silicone and other polymers. Casting silicone is similar to casting any other polymer and involves (put simply) mixing a liquid base with a secondary agent that hardens that base, and pouring it into a mold (usually coated with a release agent so it doesn't stick — make sure you use a release that is designed for medical implants). Most involve a 10:1 two-part mixture that can be cured at room temperature, although many variations exist (numerous websites describe the process in detail).
Pourable silicone for casting are available in a wide variety of durometers and should be obtained in grades specifically intended for long term implant applications (ie. biocompatible — "implant grade" or "medical grade," not "healthcare grade," which is only suitable for external use such as special effects).
High grade plastics are readily available to the public — in theory, anyone could just buy them and manufacture custom implants by pouring the plastic into cookie-cutter shapes in their kitchen. However, to avoid voids (bubbles) in the implant, a degasser or vacuum chamber (available in the $200-$300 range, although some people have experimented with $30 dessicators and other low-tech solutions as well) should be used, although it should be noted that, with some low viscosity plastics, these are not needed. Other options include technologies such as injection molding, but these tend to be outside of the price range of most non-medical implant artists. In addition, medical casting should be done in a clean environment (clean as in clean-room, not clean as in Mr. Clean) to avoid introducing surface contamination (ie. if it's in the air, it's in the implant).
In more detail, a typical procedure would involve the following steps (assuming that a mold has already been prepared properly and sprayed with a medical use release agent). Please note that this is a generalization, and you should follow the manufacturer's directions, not these!
- Mix the silicone and catalyst, in precisely the required ratio. Stir vigorously for about five minutes.
- Degas the silicone if required (ie. get the bubbles out of it). This will probably take about ten minutes.
- Pour the silicone into the mold and put that back into the vacuum chamber for secondary degassing (with some designs of chamber you may be able to do this all inside the chamber). This will probably take about 45 minutes. Hardening usually takes between one and two hours from the initial mixing.
- Depending on the silicone being used, you may have to add heat to finish the cure, although most will cure at room temperature — just not as quickly.
The resultant implant can be carved additionally if need be, and can be sterilized in an autoclave without damaging it in any way. There are multitudes of silicone suppliers (just type "silicone casting" into Google) which sell the raw materials, as well as the tools required to shape them (although they can often be found on eBay for less).
Note: There are a very limited number of implant silicone manufacturers as a result of the liabilities involved, and many suppliers for these companies will not sell the materials to the general public. Manufacturing a truly "clean" silicone implant is an exceptionally expensive and involved process.