Is that available from Campy, TA, Stronglight or others?
Would like to replace the 34.

They come in these flavors:
34-48
34-50
36-50
The one you most often see in the US is the 36-50.

A technical question:

Do you have to change both chainrings? In other words, is there a 50 chainring that is mated to the 36? Or can I just buy a 36 and swap it with the existing 34?

I'm thinking of puting an order in at aebike.com

A technical question:

Do you have to change both chainrings? In other words, is there a 50 chainring that is mated to the 36? Or can I just buy a 36 and swap it with the existing 34?

Theoretically, the pins and ramps on the outer chainring are aligned to work best with a particular inner chainring size. But in reality, the differences are minor, and most people would be hard pressed to detect a difference in shifting performance. Many people swap either inner or outer chainrings at will with no problem.

The shifting difference between the correctly and incorrectly matched 50 chainrings was clearly evident to me. Yes, the mismatched ring will work. I road a mismatched set for several hundred miles. However the matched set shifts quieter, smoother and quicker. I felt it was worth having the correct matched set, IMO. You will tell the difference with the first shift going onto the 50.