To make your own you basically need to acquire some reference data resource that you check each word against to see if it is in the resource. As for suggesting words, it is probably a matter of comparing the number of characters that match for different words, and could probably be a rather complex algorithm to have one that is worth anything.. You should search Google and see if you can find one already created that you can purchase... but use the term "spellchecker" instead of dictionary when you search.
For AS2, I've used this in AS2 and it is pretty good. It has a few little things that were strange, but I think I got them worked out.
And it is pretty inexpensive.
For AS3 there seem to be more options. Depending upon what you need to do with it the one from gSkinner (http://gskinner.com/products/spl/purchase.php) is probably really good. But a bit expensive.
Or you could make your own. Search on Norvig spellcheck to get a good primer on the conceptual underpinnings.