If you buy a bottle, how can you be sure which type you're getting? And even if it said, would you trust what they said it was?
Therein lies the rub - you don't! In this country, only isopropyl nitrate is now legal - despite the fact it doesn't give the same experience as the older isobutyl nitrite, tends to cause worse headaches, and is linked to the risk of eye damage. Many sellers will claim to sell the old stuff, but what you receive is still just isopropyl - whatever the label may say.
It appears that France has loopholes that mean that isobutyl can still be obtained. In the end, your best chance is to buy from one of the known french sellers - or find a brit who buys these and sells them on. It's all a pain really, and a lottery. But, if you're a fan, it's worth the effort.
It's a wonder anyone new actually bothers, these days...