I've got a No. 3,5 and 7 travisher. I thought that my 3 would do that contemporary deep concaving that people want but it was the 7 that came closest. Can't wait until I've got the money to buy more willow so I can actually use the thing!
I've never seen a No.18 plane. Who makes them? A quick google didn't turn anything over
I made my own conacving plane out of a cheap wooden coffin plane I got off of ebay. Fletch has done one too. The only tricky part is grinding down the blade to match the shape of the plane. After using it my view on them is that you can't actually see what you're doing under the plane but it's brilliant for moving lots of wood quickly.
Ross, ebay is where most people find their stuff. Saturday markets are good as well if you like getting up early!