Component on Crypton gas analyser circuit board
That's a KBL02 bridge rectifier. You can get them at a number of places. Mouser has them. You may need to find another supplier if you live in another country. Do a Google search for 'KBL02 Bridge Rectifier' for a list of suppliers...

To answer an unasked question... You might need to find out why the bridge failed. It could have been external (voltage spike, etc. unlikely), old age (components fail on their own sometimes) or some short in the circuit board that the bridge feeds. If the replacement bridge fails (get two- they're cheap) then check the capacitors and other downstream components for shorts. In fact, you could do a quick check (on the DC side) with an ohmmeter before putting in the replacement. If you see a dead short then further investigation will be required.