As part of a larger project I needed to make a two part silicone mold. I'm making a mold of a cassette tape, but you can follow the same steps with virtually any object, but you'll need to modify the steps in order to suite your object. I'm using OOMOO-30, which is nice because it's fairly affordable, and comes in small quantities. There are more resilient forms you can buy from smooth-on and elsewhere that would be better suited for lots of casting.
Below you can find the list of materials. You can find many of these in a craft/hobby shop as well.
List of Tools & Materials:
Smooth-on OMOO-30 Silicon: http://amzn.to/1PKoatI
Linocut Carving tools (Lot of options available, this is just one set): http://amzn.to/1OoJtlg
Individual Carving tools: http://thecarvinggloveguy.com/
Sulphur-free clay: http://amzn.to/1S3wWBB
Clay sculpting tools: http://amzn.to/1KxMCGT
Foam-core board (for mold box): http://amzn.to/1PKpb5b
Mixing cups: http://amzn.to/1MV0v3M
Glue gun: http://amzn.to/1PKprkB
Mold release: http://amzn.to/1S3xQ0N
Vaseline (cheaper than mold release, but might damage mold if used often): http://amzn.to/1PKpKMl
Drop cloth: http://amzn.to/1S3yP15
Plaster of Paris: http://amzn.to/1LYJ2Yc
Harder form of plaster: http://www.dickblick.com/products/densite-plaster/
*The amazon link are affiliate links, they don't cost you anything to use, but help keep my ink cans full.