I once was on this mission too and found that patience mixed with Ebay was the only way. Some people on there want WAY too much for an empty bottle but from time to time someone is reasonable. I ended up with a few 3L that I am filling with beer right now at the moment, a couple 6L, and ended up with a 9L. I never saw a larger than 9L for sale but that doesn't mean it doesn't happen. I'd still like to get my hands on a 12L.
If you start buying the bottles you have to be ready to do work to close them up. I am still thinking hard about making my floor corker adjustable but I'm slightly scared to cut it apart. As it stands now I just compress the cork and leave it compressed for an hour or two. Then I quickly place it in the bottle by hand. Cages are another issue. You need to make them from scratch basically.