I've seen shoes pop up in my search results
I've had that, too. Nike makes a line of shoes by the name Mercurial. I eliminated the shoes from my searches by typing "mercury vapour -mercurial" exactly like that. Most search engines will then search for mercury vapour but not for items that contain the word mercurial. It works in Google and eBay search engines, and it should work for most others.
The most annoying I find in trying to find something online is eBay searches that will list a lamp with the title along the lines of "Mercury vapour metal halide high pressure sodium lamp" (just one lamp is in the listing, though). The description reads something like "I don't know much about this lamp but it looks OK." Then the picture shows a metal halide lamp with a jet black arc tube. Obviously the seller doesn't know enough about them to be selling them, but it is what it is.
The other annoying thing is when you're searching for lamps in Google images. You then get some "garbage web sites" (that is my term for them) where they list all kinds of random images they found online, with a few sentences describing each. Those websites won't get you anywhere.
That said, I wish you all the best in augmenting your collection !