Please don´t use the word "adopt" for such a situation. You can adopt an unwanted stray/feral domestic cat or dog from a shelter. But you want to BUY a hand-reared serval kitten, very different situation.
Servals are bred by private people in Europe in several countries. The UK is one of the hotspots from where people like you source their kittens. I have seen prices 4.000-7.000 EUR few years back.
Serval is covered by CITES II and you will need official document proving the kitten is captive bred and extemp from ban on wild animal trading. You must also check the national French legislation - what sort of licence you must obtain to be allowed to keep a serval, what minimum space of enclosure you must build and so on. It can very well be that your local government/municipality has further limited or forbidden this type of wild animals to be kept privately. So better contact the relevant authorities (including veterinary office) with a request before you even try to source any animal.
Servals are not good animals to be kept in a house/flat permanently. Unless you keep it contained to rooms where you will remove all furniture, textile and other equipment that you don´t want to be destroyed. And you can bet you will smell it.
I know that you can buy also hybrids between servals and domestic cats. I don´t know details, I dislike this trend. Some of these hybrids are legally seen as domestic cats and don´t fall under wild animal legislation and CITES.
This forum is for zoo visitors and fans of official zoological collections. If you need more info, you should join private keeper forum instead.