Catalytic Converter vs. O2 Sensor replacement

I have search the past threads and I am either lost or more confused then when I started looking. Can you replace 1 and then the other over 3 months ro save up and replace both?
My question is can you replace the O2 sensor and not replace a (might be bad ) Cat converter? or upon replacing just the sensor, will this cause the new sensor to fail quickly with in 3 months = (Funding shortage)?

Also where do users suggest on getting replacement items at a discounted price?

I have a XJ92 I6 4x4

As long as your catalytic converter is not clogged, changing one before the other won't be a big problem - but it's best to change both at once.

For the HEGO sensor, I'd check the aftermarket and use either Borg-Warner or ACDelco. Nearly any aftermarket converter will be fine - but keep the bolt flange from the old unit and have it welded locally (I've got a local shop that does carry-in welds for free or close to it - but I'll always offer something anyhow...) Since it's a single-stage converter (no AIR inlet,) you have options - I don't think the 1992MY has the downstream HEGO sensor. My 89 has a BBChevvy converter with the right pipe sizes that works nicely, just passed CA smog and I picked up a little response by increasing exhaust flow (the BBChevvy muffler helps as well...)

5-90