I tried at least 8 Walmarts near me, in north Mississauga (following tip/instructions above). I tried Mississauga, Milton, Brampton, Georgetown. All of them return the same message:
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Sorry! There are no appointments available to be booked online at this time. Please check back later. Please note that Asymptomatic COVID-19 Testing is available in select locations only.
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Will keep trying. I will need 5 of those tests for the first week of December, but I don't like the fact that I can only schedule the test 3 days in advance at Costco.