Hi, I thought I'd weigh in and give my opinions...and I do feel honored that my name was mentioned about restoring a motorcycle because my progress has been SOOOOO slow.
I find that there are enough sites and people on the sites that know english and make finding parts much, much easier. I've bought 2 fenders, oil tank and tool box, gas tank (a 35 year hunt!) and a number of smaller items either from ebay.fr or lesanciennes. A big part of it is just getting lucky that someone posts exactly what you are looking for, or at least close to it.
Leboncoin has a great many parts but I have only had 1 reply out of about 25 emails I have sent, so I don't use that site much.
There are some other places that sell retro parts (listed in my posting) and a couple sell the Dunlop seat you are looking for. Unless you want to wait for who-knows-how-long for seat in great condition I think replacement parts are the way to go. When I get to that point I'll be buying replacement parts for a new seat, computer cables, brake cables and exhaust pipe.
Terrot.org is also an option, though they don't encourage the buying and selling of parts.
Hope this helps - I love your bike. Looks like a great restoration project! Joe