As far as reliable sources of lao zhuni teapots, I don't know of a single completely reliable online source. What I can tell you is that you should probably expect to pay in the range of $400-1000 at a minimum for a credible old zhuni teapot, and even then, you may not reliably be able to be assured that it's not a fake. There are probably some credible (and expensive) sources in Asia, but even then, I think getting even a 95% certainty is going to be difficult.
Of the sources out there, I would probably put Hou De as one of the more reliable sources, and if you email the proprietor privately, he may have something that would be of interest to you. But (with all respect to Guang), I don't consider myself enough of an expert to say with any degree of confidence whether his old Zhuni pots are genuine. You could also try Best Tea House in Vancouver (call via phone), but I think the prices will be quite high.
So, my other suggestion is, buy some Yixing teapots that are more easily accessible (including some more modern zhuni pieces) and develop a little more experience, before trying to buy the sort of thing you're talking about here. Even fairly average zini pots from 1960s-80s are getting fairly expensive these days. I would also suggest spending some money on one of the two MAI foundation books, which have some great pictures of some older zhuni pieces. Doing your research ahead of time is great; and I understand your desire to make the "right" first purchase, but there's a reason Yixing collectors talk about tuition.
While zhuni pots do tend to be small, partially because of the high shrinkage (and high breakage) rate, from what I can understand, I think ones in the size range you mention will especially be difficult to find.
Really, you probably need to be buying from private collectors, which means, networking, time, experience, and building those relationships. To do those things, you need to build up some trust with a vendor or collector (in both directions), which won't happen overnight.