I have been looking for a strap for my rouge H Kelly and this is the wisdom I have gleaned from a few conflicting SAs and store managers:
You can order a strap, but the wait now can be as long as two years. Even for canvas straps it is long now. H will not sell you a strap without seeing the bag, and may very well take the bag for a period to ensure a match.
Some stores (FSH, Madison) do have straps in stock, especially of current common leathers and neutral colors. They are patchy on whether they will tell you this. And again, they will NOT sell you a strap without seeing the bag.
If you have a damaged strap, however, even if it's in shreds hanging on, H will repair it