local dealer, or for probably less even with shipping onlinemazdaparts.com.
In my p5 I installed a car starter that has a timed mode to run it for 5 minutes every 3 hours since i only had enough money for either a starter or a block heater and i wanted my car to be warm in the mornings. Consider that option as a starter with a timer will run you about the same amount installed. there are also some newer models that only run when its below a certain temp.
I have yet to have my car not start even in alberta winters, except from a dead battery after being parked for 6 weeks when we had those epic snowstorms last winter (it was buried over the roof lol) but after a quick boost it was good to go.
just in case anyone else finds this thread, I found a block heater plug, made by Kat's (part 11411). they can be bought by mail order through various distributors in the USA for about 50$ plus shipping. unfortunately, the one I hoped to install will not work, as my mechanic tells me the core (or whatever the plug is installed into) is too rusted... bummer. now have to investigate other options... I have a heater pad stuck to the bottom of the engine, but it doesn't do the job particularly well when its really cold (I live in Calgary)
if anyone wants my frost plug, be in touch.