Again depending on what you need them for, there are a bunch of options. Coding your own isn't too bad, but any sort of division algorithms can get nasty in a hurry. If are doing something like does include a lot of mathmatical tinkering, an RSA implementation for example, you might want to look around for some open source packages.
MAPM is a c package
MAPM that's pretty damned nice, you might want to give it a shot, there is a C++ wrapper class for it as well if I remember correctly...