Download blocks in blk00000.dat format from a specific period
Bitcoin Core is the only software that saves the blocks in the blk*.dat format. This format is not standardized and internal (i.e. not exposed to users). The contents of, for example, blk2342.dat can differ between multiple instances of Bitcoin Core.
The only option is to run Bitcoin Core yourself, or to find somebody who runs Bitcoin Core and can provide you with these files.