After not finding and at reasonable price, I decided I'd make some. Here's what I did.
I bought two lengths of 75x50 timber. They didn't have the ash I wanted where I went, so I decided to try Hemlock. I thought Hemlock was a poisonous plant, but it seems it also a fir type of tree, grows to 60m and 2m diameter, mainly in Nth America.
I lopped 2 pieces at 600mm off the end of each, and glued these to the sides of each stick. That gave me a sort of paddle blank.

Next I marked these out for cutting...

And then proceeded to cut out the rough shape.

Then there's a lot of work with plane and spokeshave..

For handles, I smoothed 400mm at the other end into a round shape.


Eventually I ended up with two oars.


Now I've started to sand them and varnish. I'll leave the handles unvarnished.
They're just on 12ft long. I hope I never have to use them!