Many people say it’s good to varnish the top side of cane webbing, but not both sides (though I varnished both sides of all my cane seats before I heard this advice, and so far they are doing okay). Supposedly if you varnish both sides, you don’t succeed in preventing water from soaking in, and now the material dries more slowly than it could before. I suppose varnishing one side makes the material wear a little longer while still having plenty of exposed drying area.
You can search on-line for cane webbing suppliers. You can get fake cane webbing now too, which looks almost real, and is longer-lasting than the real stuff (it also doesn’t sag when it gets wet). If you have a furniture store near you, they can tell you where to get webbing, and they might even be able to install it for you (furniture places that deal with those old-fashioned kitchen chairs with cane webbing will know what to do).
You might consider a seat filled with nylon webbing instead of cane. It’s pretty comfortable, a lot tougher than cane, and is unaffected by water. I trimmed a web-filled canoe seat to fit the center position in my guide-boat, and use that seat 95 percent of the time instead of the original cane seat, mainly because it’s just about indestructable.
You can buy seats in a number of styles from any paddleshop, but lots of people here like Eds (already mentioned in an earlier reply). I’m lucky enough to have a good paddle shop in town, and have replaced a number of canoe seats with what that shop has in stock. I can vouch for the much greater comfort of a contoured seat as compared to the usual flat seat (nothing else about the two styles are different other than that slight curvature of the seat frame).
Make sure your replacement seat has the same front-to-back dimension as the original so you can re-use your existing bolt holes in the gunwales or mounting plate (if your boat has one). Sometimes you can get different seat widths in the right-to-left dimension too, but either kind will fit any boat. You ALWAYS have to trim and drill the outer ends of the frame to make the seat fit your boat.
Choosing your new seat, and trimming and drilling the ends is the same process whether your seat is mounted on hangers from the gunwales, or on an angled metal plate below the gunwales.