A couple of different places online sell them. I have seen them at Blue Zoo
and at Live Aquaria pretty religiously. Pet Solutions sometimes has them listed as well. We also see them from time to time at fish stores around here. A large LFS in Dayton called Gerber's has ordered them in upon request. They typically run in the $25-30 range. One barnacle blenny is kind of boring, so it can be quite the financial strain to purchase a group of 5-6 of these guys.
This particular species was being brought in quite a bit a couple of years ago. For whatever reason it was very difficult to find the females. We got this one female mistakenly and actually thought she was a different species for a long time until we did more research. In this species, the males seemed to fight with each other much more and sometimes killed each other so it was more of a priority to try to find more females.
We no longer have these guys now, but do have a different type of barnacle blenny called the Hancock's barnacle blenny (Acanthemblemaria hancocki.) We have seen this type coming in much more lately and was actually the original species we were trying to find when we got the Brown Cheeks. I will have to do another fish finder on these guys, but they tend to stay slightly smaller, get along better, and no noticeable differences between males and females. Here is a picture of one of ours:
Also, this is not my youtube video, but shows how all of the barnacle blennys act. Notice how they are playful and constantly switching barnacles into each others. It's really fun to watch them interact.
One more thing to mention, all barnacle blennys are meticulous cleaners. They hate anything being in their barnacles (or their hole if they aren't using a barnacle) including food. I have seen food accidentally float into a barnacle and the blenny will quickly remove it and drop it outside. Then it seems to hit him that that was a food and he will shoot out and eat it. But obviously, having a clean den is much more important than eating.
It is pretty inevitable that they will normally find holes in the back of the tank where you will never see them again. I find the best way to get them in your tank is to acclimate them in a covered rubbermaid dish. While they are floating, put the barnacles in with them. They will take right to the barnacles and all you have to do is lift the barnacles out and position them in the tank. Once they are in there and feel safe, they for the most part stay in that vicinity.