sx get-pubkey ADDRESS

If the address is available on the blockchain and has already
a spent transaction, the pubkey is retrieved.
Multisig (BIP16) addresses are not supported.

tested:
$ sx get-pubkey 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa
Address 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa has no spent tx so no public key is available
$ sx get-pubkey 3A5XQWZPzG4GrpVeYFaDBZ1s4sXkYB69Wf
Cannot get public key for script hash based address 3A5XQWZPzG4GrpVeYFaDBZ1s4sXkYB69Wf
$ sx get-pubkey 1EXoDusjGwvnjZUyKkxZ4UHEf77z6A5S4P
04ad90e5b6bc86b3ec7fac2c5fbda7423fc8ef0d58df594c773fa05e2c281b2bfe877677c668
bd13603944e34f4818ee03cadd81a88542b8b4d5431264180e2c28
$