Can anybody tell me what the spikes are like that come with the ERR?  Pictures would be welcome and appreciated, too, if possible.

The studs are indeed 1/4-20. Half inch or 3/4 work fine. I put 3/4 versions of the Herbie's Gliders on all mine. The supplied spikes are a very solid, somewhat squat shape, and nearly black in color. Not using mine. When I return home today, I will be happy to post a pic.

Here is a quick snapshot of two of the spikes:


They are a bit less squat than I had remembered. From tip to base [without the stud], they are almost exactly 1 inch in height.

1/4-20 thread. One could consider this standard for this application.

I wondered if a person could start with some 1/4-20 SHCS and just screw them into the threaded holes. Another idea would be 1/4-20 studs and lock nuts.  From what I have seen of the spikes is if you have to pay more than $4.00 for them, someone is taking advantage of you. Nothing but some hex stock with a couple of minutes of machining on them.
To answer your question, 1/4-20 is the same on everything you will ever run into. Sometimes they will be called 1/4-20 UNC. Same thing.
You don't want a 1/4-28 UNF or a 1/4-18 NPT or NSP.

Just to round out the data set, the supplied spikes have a mass of 33 grams each or approximately 1-1/8 oz in weight. Donnie is right, nothing fancy, just a nice hunk of pointed metal designed to decouple the speaker. The fact that they are included with the ERRs is just another value indicator to my mind.