Hi amdefacto,
As you are also aware of the fact that there are almost 'No' really good Free spellchecker controls out there. Here is a list of few paid ones which I can recommend
http://www.telerik.com/products/aspnet-ajax/spell.aspx - 999 USD
http://www.devexpress.com/Products/NET/Controls/ASP/Spell_Checker/
http://www.aspnetspell.com/asp.net-spell-check-control.aspx
Also I've heard of some people who did use Google API to do the spell check, so if you want to try building it you can try that way.
Regards,
Chinmay