You can use Bitcoin Core to obtain that data. Here's an example,
$ getblockhash 700000
0000000000000000000590fc0f3eba193a278534220b2b37e9849e1a770ca959
$ getblockheader 0000000000000000000590fc0f3eba193a278534220b2b37e9849e1a770ca959
{
"hash": "0000000000000000000590fc0f3eba193a278534220b2b37e9849e1a770ca959",
"confirmations": 84720,
"height": 700000,
"version": 1073733636,
"versionHex": "3fffe004",
"merkleroot": "1f8d213c864bfe9fb0098cecc3165cce407de88413741b0300d56ea0f4ec9c65",
"time": 1631333672,
"mediantime": 1631331088,
"nonce": 2881644503,
"bits": "170f48e4",
"difficulty": 18415156832118.24,
"chainwork": "0000000000000000000000000000000000000000216dd8dc61fdffabb624feeb",
"nTx": 1276,
"previousblockhash": "0000000000000000000aa3ce000eb559f4143be419108134e0ce71042fc636eb",
"nextblockhash": "00000000000000000002f39baabb00ffeb47dbdb425d5077baa62c47482b7e92"
}
$ getblockheader 0000000000000000000590fc0f3eba193a278534220b2b37e9849e1a770ca959 false
04e0ff3feb36c62f0471cee034811019e43b14f459b50e00cea30a000000000000000000659cecf4a06ed500031b741384e87d40ce5c16c3ec8c09b09ffe4b863c218d1f282d3c61e4480f17d767c2ab
Although if you don't want to download whole Bitcoin blockchain, you could just get raw block data from block explorer and extract raw block header manually.