Are there any blockchain explorers that provide raw Tx data and block headers?
You can get this info from blockchain.info, by simply adding ?format=hexto the end of the relevant URL. 

For example, here is a recent block (height 509,244):

https://blockchain.info/block/0000000000000000000635bda771916ca727db53fea5441508f7161386e066be

And here is the raw data for that block, in hex:

https://blockchain.info/block/0000000000000000000635bda771916ca727db53fea5441508f7161386e066be?format=hex

(Note that you will need to parse the block header out of this data, as that URL will dump the raw data for the entire block)

Similarly, here is a transaction from that block:

https://blockchain.info/tx/74d350ca44c324f4643274b98801f9a023b2b8b72e8e895879fd9070a68f7f1f

And here is the raw data from that transaction, in hex

https://blockchain.info/tx/74d350ca44c324f4643274b98801f9a023b2b8b72e8e895879fd9070a68f7f1f?format=hex