Where to get it
The newest version of TTDPatch should always be available from the TTDPatch home page.
If the page doesn't exist, it has probably moved. But even if or when it does, you should still be able to find it in the Transport Tycoon Webring.
Because TTDPatch is distributed under the GPL, the source code is available too, at the TTDPatch web site.
It can be compiled with free compilers, using NASM, Open Watcom for the DOS version, GCC and various other tools from the GNU development suite.