I am having trouble fining a strain relief clamp to cover the hole into a metal canister (the canister for a heating unit). Is it ok to use an NM cable clamp - the kind you use around the hole going into a metal junction box (where you tighten the clamp by turning the 2 screws)? Is this considered a strain relief by the "average" inspector?
Since your can has a standard 1/2" knockout, this is what the NM clamps are designed for -- it's simply an odd-shaped box from the Code's perspective.
You need to use strain reliefs/clamps which are correct (i.e. listed) for the cable you are using. For instance if you have cordage (cable designed to be a flexible appliance power cord), don't use a strain relief intended for NM simply because it's the only thing Home Depot sells.
Home improvement shops like Home Depot or Lowes have notoriously bad selection in this sort of "oddball stuff"; they do not want to stock it because their business is all about volume. A proper electrical supply house will have the right strain reliefs; bring them a cable sample and they'll put you right.
As an aside, you need to use flexible cable that's correct for the usage (assuming flexible cable is allowed at all). The most common mistake is using NM cable for cordage. Again, the electrical supply will have a variety and can also advise you.