I have the dotline adjustable rod holders and they are awsome. Perko made a version very similar years ago and I looked on E-bay for them, and then the DOTLINE became available. I only caution that there are some a couple of washers and a thumd srew bolt that could be lost overboard. The blue water series is new and looks like it has less parts. Do a google SHOPPING search for adjustable rod holders. Good Luck,
Boater's world also has them.
http://www.overtons.com
Dotline Deck-Mount Adjustable Fishing Rod Holder
Blue Water Series Removable Adjustable Stainless Steel Fishing Rod Holder