OK Adam I had a few minutes yesterday so here are the floor images. No worries if you find you have the stringers because it might help someone else searching in the future. You'll soon know if yours should have been wood because , as you can see in my images, there are lengths of the same section glued to the end boards.
Here is an overall view of all the floor sections with one stringer on and one off.
A close up with the stringer fitted.
Similar with stringer off.
And a close up of the stringer section.
With respect to Prairie Tuber I'd say mine are not the woods he mentioned as there is no redness to them. I'd go for birch, white, oak, ash, maple or similar.
If you did need them be very interesting to hear if Zodiac can still supply and how much. If I needed some I'd go to my local woodwork workshop where they have router machines that can make any shape. From work the guy has done for me on other projects I'd expect to pay about £40 for a pair including him supplying a suitable wood. Taking in one of the existing boards with the glued on section would give hime something to copy.
By the way some folks say these stringers are hard to fit... but it's just doing it the right way that's needed. The boat needs to have air in it enough to about 50%-90% fill the tubes but with no pressure. Then importantly you need to put the oars or something similar under the boat lengthways about 6" in from where the edge of the floorboards are so they are supported as you fit the stringers. Do this and the stringers will just push and roll into position 10secs a side.