If you are planning on buying insurance to cover the time in Canada prior to MSP coming into effect, you should buy it BEFORE arriving in Canada. That way, there is no waiting period on coverage related to sudden illness. There are no companies that 'give you a few days to this'. If you wait until you are here to buy, there will be anywhere from 48 hours to 7 days waiting period for coverage to begin for any expenses related to emergency illness - there is usually no waiting period for accidental injury coverage.
You should always look at the widest selection of policies in order to ensure you are getting a fair price: BestQuote Travel Insurance Agency has more policies online than any other comparable quote resource for visitors to canada :
https://www.bestquotetravelinsurance.ca/visitors-to-canada-insurance
Even more than Haver's recommendation!
Also, although it may be an MSP requirement for you to wait the remainder of the month and then two more months before coverage can begin - you want to get your application in quickly (as soon as possible) to prevent any delays in receiving your card back - without the card, you'll find it too hard to get services without paying (have you tried going to a medical clinic without a care card? And then having to say 'I haven't received my card yet'? You won't have much luck with that.