If you often confuse "defense" and "defence," you have a good defense, since they are actually two spellings of the same word. We explain how they're used.
Hunspell also has an en_CA dictionary for us (as we tend to have a mosaic of British, Irish, Scottish here more so than the States). It will, say, include both defence/defense, and will honour only the "u" spellings of most words, but not all (e.g. colour / coloration...