Those are Paul McCobb-designed chairs.

The feet are pretty much a stock piece you can find in a good home improvement store. I had some vintage bar stools I needed new feet like these for and I found them at Menards. They had a bunch of diameters, you should be able to find one that fits.

Wow - great steal, and great looking chairs.

Here's a reference that confirms that they look like they're Paul McCobb: http://www.deco-haus.com/7.html Scroll down 'till you see yours.

Note that the "Category E" pricing means that their set of four are going for "under $1000".

...upholstery looks original too.

along with a hardware store, any good upholsterer should have these ..I had some chairs re-finished and he changed out the feet as well...

This company carries a large selection of mid century furniture parts:
http://www.swivel-chair-parts.com/index.html

I loooove those! Good find. Try ebay too... I see replacement feet listed on there all the time :)

Decent hardware shop sells these feet, in different sizes.

However, you won't find them in black-- they'll be brass-toned with white silicon bottoms. Several coats of black spray paint outta do the trick.

Oh... I now see that the originals are brass with a patina of age.

I'd recommend buying the brass-toned ones, then applying whatever accelerates tarnish on brass.... but I KNOW that the feet commonly found in hardware shops aren't solid brass.

Nice job finding these for $25! I was stalking them on ebay for a while this fall, and they usually go for around $150 each.