Twice I’ve ordered things through Amazon.fr, strimmer cord and saxophone reeds, and they’ve arrived posted from the UK. The vendors covered the extra postage.
this site might be interesting for you…
Liam you made no comment so I assume you missed this amongst the forum noise
Is the diatomaceous earth you use food grade…??? If so then I use it too both as a dietary supplement and if necessary in the coats of my Border Collies and added to their raw food to guard against internal and external parasites…and haven’t yet found a cheaper alternative here…I get mine from wiggly wigglers in UK…x 
Just wanted to mention that John Withall’s suggestion to try the phrase “produits chimiques” worked well for me. I don’t search using Google; I use DuckDuckGo’s search engine called DisconnectMe (https://search.disconnect.me/). It’s much more private. I found Onatera’s website (https://www.onatera.com/), where I found Epsom Salts which I’d been searching for to no avail in local shops and supermarkets in my area. I confess I didn’t try searching for the other chemicals Liam was asking about.
Thanks, John! And many thanks Liam for bringing up this topic, so I could find something I’ve been looking for, for over a year. Cheers!
For those looking for epsom salts, Action often have them, as well as a lot of other chemical things in their hardware section.
Looks like you will be able to get hold of neo-nicotinoids fairly easily.