where can I find the source code for tftp (Trivial File Transfer Protocol) ?
tftp is a protocol and not a piece of software. It has no source code itself but there are many open source implementations available.
TFTP is a protocol. The specification is available in RFC 1350.
If you want source code for an implementation, a Google search for "tftp source code" finds a lot of answers, among them [TFTP].
tftp-server package of CentOS / Fedora / RHEL is based upon: