Hi Simon
I’m also a relative ‘newbie’. We have an excellent club spares department (reachable from the TOC home page) but you can also purchase from several continental suppliers. Most of these do supply body panels and sections though they are not particularly cheap (the euro is not helping at all) so it would be up to you to decide whether you would be better off making the parts fron scratch. I have used 3 of these so far and found the quality good and delivery fairly fast (up to a week)
http://www.renelauto.fr/catalogue2016.pdf
https://www.citroen-traction-avant.com
https://www.ctaservice.nl/your-products/18039/TA
http://depanoto-boutique.com/
https://www.ctaservice.se
Cheers
Paul