Ditto on Stabilized Marine. Make sure you tell them which system model number you have. Also you can call Naiad and see if they can refer you to other high volume/servicers of these. I don't know if he is still n the biz but Gary Sorenson of Marine Controls did a lot of Naiad work for a number of large yards on the east coast, and may be able to point you in the right direction. He did a variety of hydraulic work on my boat, good guy. 828-508-4201. The fins are pricey (probably around $2500 each for 6 footers) bought new from Naiad, so take outs are the way to go if you can.
"There's the Right Way, the Wrong Way, and what some guy says he's gotten away with"