Any regiment not registered in the forums is available, as a rule of thumb. There are countless minor regiments without a thread on fse but they often form and disband so sporadically that it's impossible to keep track of them all.
Just search through the regiment section here and find a name that isn't taken, and then make a thread for your own regiment. If you see another person in-game who is using your name you'll have the right of way over him, and you can either convince him to merge with you or change it accordingly.
Thankfully in BCOF all this nonsense will be sorted out so we won't have to deal with people stealing names and stuff.
