Try
SpellBound. And just in case, here's a
Tutorial
BTW - the SpellCheck native to A2K is pretty good - it has some quirks, but then they all do. To use it, compose your post from "Post Reply", instead of using "Quick Reply", or if you do use "Quick Reply", click "Preview" instead of "Submit" - that will open the "Post Reply" page with your post as it will appear up in the top panel, and editable text below, in the Post Reply dialog box ... the spellcheck button is at the bottom right of the text input dialog box - play with it a bit to learn how it works. If you want, you can also compose your post in an external text editor, paste it into the "Post Reply" dialog box, and spellcheck-and-correct that way.
Or, like some folks we know, you can just go with the typos and let other folks have the fun of pointing them out for you