At times when Doom9 and Sourceforge won't load, I see if I can get there using MyProxySite.org - surprisingly, it actually works most of the time (although sometimes the sites are just down, period).
Doing so, I found this directory:
http://members.optusnet.com.au/squid_80/
I think the version of AviSynth for 64-bit might still be 2.5.6, though. I couldn't see any verification on that. I also don't know if it's a case of switching out the DLL or anything like that, considering I don't have a 64-bit OS to work with (save for Ubuntu 8.10, but I'm not about to try to get this stuff working with Wine).
Alternately, I'd just see about getting the regular 32-bit VDub and if that works, then all the 32-bit codecs should work as they're meant to.