I emailed DI a while back and asked him the same question. This is the email he sent me

Hello Wes, the holders i got came from Mark Danque of The Crappie Hole in SC. His number is 803.345.5606, its worth going up to see him, his shop is second to none when it comes to crappie fishing.

I didn't go with him cause it was a little too far for me. DI rigged his entire boat with this guy for around $300, and DI has a total of 30 rod holders on his boat.