Where can one download Bitcoin Days Destroyed data? Alternately where to get data to calculate BDD?
For Bitcoin Days Destroyed data you can for example go there https://blockchair.com/bitcoin/charts/coindays-destroyed?interval=3m you can download the data they used in .csv

https://blockchair.com/bitcoin/ does a pretty good job at providing useful data, charts about the bitcoin blockchain

If you want to compute and made your own calculation yourself the best way would be to download the full blockchain 250giga+ with bitcoin core https://github.com/bitcoin/bitcoin and use a tool like Abe https://github.com/bitcoin-abe/bitcoin-abe to configure how the data are treated