Depends on your definition of executable, in this context.

If you mean files that you (current user) are allowed to execute, then you can use something like this (finds non-executable regular files; no directories)

find . -type f ! -executable

If instead, you mean files which have the +x permission set (for their owner or group or everyone), then i believe you want something like this (finds regular files; no directories; which do not have u+x, g+x nor o+x)

find . -type f ! -perm /0111