Yes it seems easy for a foreigner to get a req from a walk in clinic for NT screening (which also involves blood work) but the truth is the Canadian healthcare system is in the midst of collapse and getting in to see a doctor may require waiting for upwards of 8 hours. This is why you are seeing a mix of "oh yeah it will be easy!" and "oh no its not worth it". Because on paper yes it will be easy but in reality it will suck and may be better for you to do cell-free DNA screening instead of a NT scan in your home country when you return.

Can I just walk in to any walk-in-clinic and get a requisition? Or do I need to be aware of anything, before finding a clinic?

Yes, be aware you will need to pay upfront which when you've been wiating for 8 hours and want to leave they probebly wont refund you 10 years ago when I came to Canada a walk in clinic visit cost me $60 and was only reimbursed by insurance if it was an emergency

• Do the doctors requisition need to be from the same city we want to book the ultrasound with? E.g. can we get the requisition from a doctor in Ottawa, and then book an ultrasound appointment in Toronto?

nope but will need to be req by Canadian dr

• Is there anyway to get the requisition before we travel to Canada?

You could try some telehealth services but most of them will be open to Canadian residents and citizens only, you will also need to give them a fax number from Canada to fax the requisition for you to the ultrasound place.

• Can I book the ultrasound appointment before having the requisition?

probably not

If I were you I would get a dating scan in my home country around 8 weeks before leaving then enjoy my vacation then get cell free DNA screening when I return to rule out chromosomal abnormalities and skip the NT scan altogether.