This is the wise thing to do.. It is good to learn or experiment as long as possible instead of just jumping in to trade with hard earned money and regret later.
There are lot of them but I will recommend a few quality ones:

HitBTC Demo trading
https://hitbtc.com/

Cpaytoday Exchange Simulator
https://www.cpaytoday.com/trialbitcoinexchange/

Niffler.co Learn Crypto Trading Simulated Crypto Traders
https://niffler.co/

BitMEX Stimulated Trading.
https://testnet.bitmex.com/