I wouldnt use a canoness, but if you are going to, then forget about the Combi Weapons.
Give her an Eviscerator. Stick her in a squad of Celestians. Have the Celestian Superior take any necessary challenges while the canoness goes to town with her sword. Its a decent enough counter attack unit.
If you want to get combi weapons, get combi flamers. You can get enough basic meltas that one more is somewhat redundant. A combi flamer however will give you the option of not only shooting it, but using it for a Wall of Death which acts as a protection against would be chargers.
This model:
Comes directly with a combi flamer and can be purchased on Ebay from 10-20$ apiece. It's a discontinued model, but a beautiful one (my opinion) and will save you having to convert them. I manager to snag 3 myself and it didnt cost me more than 40$ total shipping included... which is decent for a discontinued model.
For any other combi weapons, you will have to convert as has been mentionned.
http://chapterhousestudios.com/index.php?route=product/product&filter_name=combi&product_id=150
Chapterhouse does sell conversion kits, there are a few other companies out there that do as well and might even be better, but its a place to start