In the interest of keeping peace in my humble home, I have been on a search for some time for floor protectors to use with speaker spikes. I have seen lots of choices on websites in the UK, but relatively few in the US, and the one's I've found on this side of The Pond have been expensive. Anyone got a suggestion about where to get some brass or other metal floor protector disks that will support speakers weighing about 40 lbs.?

You can try Parts Express @ 800-338-0531.
They carry spike kits so they might be able to help you.
Outlaw supplies spikes and discs for their subs so you could might give them a call too.

Madisound has a few... http://www.madisound.com/catalog/index.php?cPath=404_121

I got some back when we moved for under my Studio 60's, and I think they came from Audio Advisor, but I don't think Audio Advisor still carries them. I got some isolation feet from Parts Express that came with dimpled discs that would make good protectors (used to separate the 980H from the 2150 in my two-channel setup so the 2150 could get better ventilation) - they may have something similar that doesn't include the feet.

http://www.myesound.com/Page3.html

These are made by Grant Vandermye, a fellow who makes aftermarket stands for Magneplanars. The floor pads on the left are $4 a pop plus shipping. Note that he also makes round bottom "spikes".

Grant's stuff is absolutely top shelf.