How to get a non-changeable bitcoin address. I tried to register on few bitcoin wallets websites but on each website btc address is changed after one payment is received and I didn't find the option to create a non-changeable one. I know that for privacy is much better to have a changeable btc address, but I need to create a non-changable btc address
With the majority of wallets, you'll be presented with a new address for each payment. As you mentioned, this is important for your financial privacy, so it is a standard practise.
That said, you can re-use an old address, and your wallet should detect the additional transactions received at that address, updating your balance accordingly.
In short: your wallet is a collection of addresses, it is not just 'the address that is currently displayed'. So you can re-use old addresses from the wallet if you so desire.
Whilst what you say is true for many online wallets, it is not the case predominantly for desktop (or server) installed wallet software.
In Bitcoin Core, you generate a new receive address, set a label if you like, and forever send BTC to that address.