In all honesty, sellswords are pretty much a dime a dozen. Given the rapid expansion that's resulted since the Dark Ages ended, most mercenaries are employed. Those who aren't, for whatever reason (as any person venturing outside Fort Vehl can tell you) have turned to banditry.
There are very few organized mercenary groups, for the simple reason that they make rulers nervous. Once a group gets beyond a certain level, it goes from "organized mercenary group" to "rogue army". At that point, rulers tend to see to it that any organization ceases. Or they hire them. One or the other. Either way, it means that short of starting your own mercenary organization, you're not going to find any official groups out there.
(This doesn't discount joining a ragtag group of mercenaries on a CDQ or some similar excursion, but outside of that, you'll have to form your own.)
