Thinking about writing my own software to calculate my taxes.
I guess you can find it at coinmarketcap in every coin you need to know about the history are listed on coinmarketcap history. go to the coinmarketcap then find the coin you want it then go to the history and click you can now check database history.
Yes, it's all there in the coinmarketcap where you can find all the current price of almost all altcoins. Also, the marketcap, the volume(24h), circulating supply and the history price chart per coin is also plotted on this database.
You can do your monitoring of it's price value at your convenience.