Where to buy and extender for an O2 sensor

Most any auto parts store. Ask for a Spark Plug Non-Fouler # 42002.

You can use a  small step bit to enlarge the hole to 1/2 inch.

How much does it cost? $2.49 + tax for a package of two.

MOTORMITE - NON - Fouler  42002

Normally the sensor before the catalytic is the one has the most effect  on the fuel ratio.

42004 is a spark plug non-fouler that when the hole at the external thread end is drilled out to 1/2 inch it becomes usable as an Oxygen extender. Available at many auto parts stores for around $2.49 + tax comes two in a package.