Considering all the various weaponry that has been dumped into Ukraine the last few years, there will be a massive black market supply soon
I'm still waiting to see a police van blown up by a javelin in a random chimp out, any day now.
Legally in France, basically :
-D category : air rifles, crossbows, bear sprays, tasers, rubber bullet guns, few other things. No restriction when buying (for anyone in good standing with the law), but need to be declared. Could get you in trouble if you carry it around / in your car.
-C category : double barreled shotguns(SxS and O/U, or even those triple barrel now that I think about it), semi-auto shotguns (2+1 shots), pump shotguns (4+1, rifled barrel only), semi-auto carbine (2+1), manual action (typically bolt action) carbine (9+1). Hunting license required, but it's very easy to get and under a 100€, for life. However you can have it revoked for very little, for example a simple
accusation of domestic abuse, even if it's retracted and not followed by any pursuits. I've heard it happens to some men with a scorned ex-wife...
-B category : all of the above + handguns and a little more lax on magazine sizes. Only for licensed collectors or licensed sport shooters.
-A category : all the rest, from anything full auto to a nuclear bomb. Illegal.
Everything has to be registered to your name in a state database.
Illegally :
There used to be a lot of WW2 weapons still around some decades ago. I know older guys who played around with MP40s,1911s and even some grenades in their youth. Most of those have been either been grabbed by authorities or have fallen in disrepair over the years.
Like others have said you can ask your local kebab shop entrepreneur what he has on the special menu. Mostly AK variants from what we see in Marseilles, and various handguns. Ammo might be the trickier part.
Last option is to learn to make a Sten in your basement and if anyone asks, just say you're doing hands-on research on the Resistance.