I posted this a while back during another discussion.
I wasn't going to bring it up again because I don't want to make anyone feel bad.
But.....since you asked.........
You can get them from Grizzly in boxes of 10 for under $3.00 each.
http://www.grizzly.com/products/h7319
I made up a bunch of roughing tools with these and call them Ozark Hoggers or Ozark Roughers. They remove wood like a hungry monster and the cutters last a very long time. My total cost for making a tool is about $5.00.
Since I bought mine, Grizzly has added many more styles of carbide cutters, some more costly. Just search their site for 'inserts' or 'carbide inserts'.
If you search for originating sources, you will find the prices are not much, if any, better than what Griz sells for and they use tech speek which is often difficult to translate.
You have used yours so you know what they can do. I'm majorly impressed with what it will do. I have found that certain woods, like Osage Orange, because of the high silica content (fancy word for 'sand') can dull these cutters. But, being so cheap and with four sides....so wat?
So far, I haven't sold any, just give them away. But, I'm looking at a new idea and may have a homemade product available before long.
BTW, round cutters are available. But, for some reason, they are quite a bit more expensive, about $12.00 per cutter. Still cheap since you could probably get a years hollowing from one cutter.