I really am not sure if this is a joke or not, but almost every major library has a C interface. Almost all packages in your distro if you use linux have development versions that ship libraries that usually are linkable by C. In fact the wealth of libraries usable from C is usually noted as its main strength.
What exactly are you missing, and have you tried googling? The GNU organization and similar all produce libraries(like https://www.gnu.org/software/gsl/) that are mostly usable from C.