I made my own, and for a time sold a few. They are similar in concept to the Boreriders, but mine have the Delrin inserts surrounded by brass made from cartridge cases. I believe mine are better because they are a bit tougher.
In a brief moment of fame, mine were featured in an article in Single Shot Exchange, along with the Boreriders and one or two others.
Frankly, I got tired of making them, and they are so labor intensive that I couldn't make enough money to make it worthwhile, although I have made a few on request. The Boreriders seem pretty pricey, but it comes down to supply and demand, so more power to him.
Paul
Mine: