A simple solution is to check your bank statements (assuming you used a bank account to receive the money) since it must be kept X year(s).
If you can access your account online, banks usually have a search function. The good thing is that as it happened in 2021, you wouldn't have to search much.
If you used cash, use any price that suits you lol, since they can't control it.
In the worst case, you estimate a period at which you have sold and compare it with the price by looking at coinmarketcap (I know it's a lazy and a bit risky method but that's what I would do as the last option). Otherwise you really need to find more history, from your wallet, your account on an exchange or whatever.
If you give us more information we can give you more directions to help
the usual pain with the taxes