I have a york 2 stage heater model 36c03 that needs to be replaced. It is emitting carbon monoxide. I can't find anything online even similar to what I have and I'd like an idea of the cost. My York was installed in 1979. Any ideas on what I should look for?

Are you planning to replace it yourself, or have a pro do it?

Have a pro do it, but I was curious of the price. He had said it was old and parts couldn't be found or hard to find, so I thought I would just replace it (them), I have 2 units, 1 upstairs & 1 downstairs.

Prices vary by locale so it would be impossible to help you there. However, I would demand a Manual J (aka load calc) before any recommendations. I would get quotes from several contractors spanning various brands and get quotes to replace both or just the one furnace.

Check back after you've done the above if you need more advice.