Not sure if you're still looking for this but I use a spell check control from telerik in all of my ASP.net apps that I developed. I think I paid around $200 for this control but it is very slick and you can down load a free 30 day demo to try it out before you purchase.
I checked into it with telerik tech support and found that it can be integrated into DNN but I have not tried it myself yet.
Here are the directions I received from telerik tech support:
If you want to have spell checking in the html editor then you will need to edit the ~/controls/TextEditor.ascx file, which contains the html editor declaration.
Copy the spell dll file in DNN's bin folder and the RadControls folder to DNN's controls folder. You need to set RadControlsDir="~/controls/RadControls" because DotNetNuke has a built in URL rewriter, which will interfere with the spell dialogs if you place the RadControls folder in the default location ("~/RadControls/").
Alternatively, you can use our custom Html editor provider for r.a.d.editor, which has the r.a.d.spell spellchecking already integrated.