All vaginal rings require a doctor's prescription. Since vaginal rings contain hormones, they may not be suitable for you if you have cetain medical conditions. Your doctor will explain this in more detail. Once you have a prescription you can purchase them from your local pharmacy.
The NuvaRing is a vaginal ring that costs around EUR 20-30 a month. If it is not available, check if there are generic versions such as Teyla in stock.