So the best source I found was mostlymetals.com. It's run by an alt-process printer who sells as "nukey235" on eBay, and it started as a buyers club so people could make group orders in quantity, but he said that it was difficult to coordinate the groups, so he started just making the large orders himself and selling it, at what I'm guessing is a pretty low markup, given his prices. I won 3 g for $60 on eBay and asked if he had more at that price, and said he would sell it at an even lower price, depending on quantity. So I purchased 25 g at $17.40/g last week (and the price is going up), and it arrived today. I haven't toned with it yet, but it indeed looks like 25g of gold chloride and not, say, orange sugar crystals.

He also sells precious metal salts for pt/pd and other processes.