Are there any free implementations of strcpy_s and/or TR24731-1?
Try with the Safe C library. It's under the MIT license and according to this list implements the functions you're looking for:
The Safe C Library provides bound checking memory and string functions per ISO/IEC TR24731. These functions are alternative functions to the existing standard C library that promote safer, more secure programming
You can use memcpy and memset etc, which are portable and safer than string functions.
Why not using strncpy and strncat? Contrary to strlcpy and strlcat, they are standard C functions. These functions are not perfect but you can use them safely.
And also note that the bounds-checking _s functions are optional in the current Standard.