Nexgard spectra! Where to get it for cheap?

Hi everyone.

I am in Canada and was going to get nexgard spectra for my 2 dogs, unfortunately from the vet it’s 306+tax for EACH dogs 6 pack. While I can afford that, I don’t want to spend that lol.

I’ve seen some websites for much cheaper, as well as Costco. If I can find something cheaper than Costco I’d love it.
I’ve heard some of these websites aren’t “LEGIT” so that’s what I’m scared of.

Any recommendations?
Thanks!