If you want to get ready-to-use C code, this is a good header-only C library for sin/cos:
http://gruntthepeon.free.fr/ssemath/
If you want to see how to write it on your own, then the good source is cephes library:
http://www.netlib.org/cephes/ (check the single.tgz file). The above SSE code is made from this one, but doesn't include all kinds of different extra C functions.
Another option is fdlibm:
http://www.netlib.org/fdlibm/ If I'm not mistaken this has better precision than cephes, but a bit slower. Most C runtime math libraries (like in GNU libc or musl) are derived from fdlibm.