You can get some Formalin MS from petsolutions.com. I believe most people here have used Kordon's Formalin 3 in the past, but I cannot find anywhere to buy that from anymore...
The Formalin MS is a 37% formaldehyde solution, however, which is also the same amount of formaldehyde that Kordon's Formalin 3 used. I would probably call the manufacturer and ask them on suggestions of how to properly treat Brook with it.
Here's a direct link... hopefully RC will allow this link
http://www.petsolutions.com/C/External-Parasite-Ich-Fish-Medications/I/Formalin-MS.aspx
I also know that "Quick Cure" which is cheap and sometimes at your local walmart contains some Formalin in it, but you would may need to adjust how you treat it to get the correct Formalin concentration. I'll let others chime in on how this might be properly used.
