The battery in my Fairphone 3 is starting to die, 12 hours between charges. I am at the present time in Canada where can I buy a new battery. Fairphone does not sell to this country.
I added this important bit of information to the topic title, before too many users browsing the topic lists of the forum just think “Then buy one!” 
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(Fairphone have it in stock currently.)
And shipping such batteries is notoriously difficult for safety reasons.
You could ask resellers of other Fairphone parts with shipping to Canada like clove.co.uk or vireo.de whether they would be willing to get the battery from Fairphone and ship it to you.
If all else fails, you could try to prolong the life of the battery by cutting down on use (if even possible), or by cutting down on stuff running in the background on the phone.
Enabling the battery saver should be an option to try, too.
Anecdotal: My Fairphone 2 batteries went from often not lasting 12 hours when entirely idle to possibly lasting for days when idle just by switching to an OS without Google services or microG as well as only installing a minimum amount of Apps to cover my most needed use cases.
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