These are always hit or miss to find locally. However I have ordered from reeftown in the past and they have always been really good.
My 500g is getting a bit overrun again now too. I added Berghia years ago, and they did great for many years, the tank was big enough to sustain a small population for a long time. But I recently added a corris wrasse and I think he ate the remaining population, so now I am going to have to try a file fish or something.