I’m re-hanging the canoe seats.
Both canoes are Canadian/US Imperial standard (as are most canoes) something odd.
New M4 bolts, countersunk are going in.

The washers however just aren’t a great fit (I’ll take that up with the supplier).

Can anyone suggest a supplier of wide-flange cup washers?

All the googling seems to find are standard cup washers.

I could also do with some hard plastic or metal tubing big enough for an M4 to fit through.

Edit: metric versions of this

They look like fairing washers for motorcycles. Might help your search.
As an aside, are you sure the bolts are M4? They look bigger (or you have tiny hands) and 4mm sounds a bit weedy for holding anything likely to carry body weight.

When I was working for shopfitting company back in the day then the go to place for specialist fixings / ironmongery was Hafele.

https://www.hafele.co.uk/en/

Failing that then a chandlers?

https://www.duncanyacht.co.uk/

search for Gunwale washers:

https://www.kentcanoes.co.uk/item-endless_river__gunwale_washers.html

The Endless River washers at Kent Canoes is what I have a few of – they’re the imperial and 34mm diameter (too wide for some of mounting areas).

I’ve remeasured the new bolts – they are M5. The supplier was quoting A2-70 at me which is the metal type…. The old bolts are closer to M3.

I’ve found some Motorsport things e.g.
https://www.ebay.co.uk/itm/274342438758

A2-70 yep a type of stainless.

Yer motorsport/bikes is a good source.

if you want stainless:
https://www.pro-bolt.com/stainless-steel-countersunk-washers/ though depends on how big an outer dia you need.

larger outer dia in aluminium alloy:
https://www.pro-bolt.com/aluminium-countersunk-washers-m5-19mm-od-pack-x5/