Buying a Cornwall to London Night sleeper ticket from Canada, for May, 2020

You can book sleeper tickets through the GWR website. Enter your origin and destination stations (London Paddington is the correct London terminus for the Cornish sleeper), and set the time to late in the evening. The sleeper train will be highlighted in the search results. The fares which allow berths (beds) to be purchased will also be highlighted — note that flexible fares like Super Off-Peak Singles/Returns have a supplement that must be paid for a berth, whereas inflexible fares will either be for a seat or for a berth. Make sure you know whether you're paying for a berth in a compartment (ie a bed), or just a seat. If you have an odd number of people, you also may be given the option to share a compartment with a stranger of the same gender (a compartment has two berths arranged as bunk beds), or to pay more to get solo occupancy of a compartment. Note that the sleeper does not run on Saturday evenings.

As for delivery options for people living abroad, GWR have e-Tickets, which you can pick up using their mobile app, or if you prefer you can instead opt to print a physical ticket from a machine at your station of origin when you arrive for your train — bearing in mind you will need your original payment card to pick it up. I don't think GWR will post tickets abroad but I might be wrong.