Most of the auto parts store do not stock them any more. The only one local is an "Advanced Auto", and they have to order them.
Champion D4 (504)
I normally get about 6-8 hours of good run time.
When the starting gets a little harder it is time to change, all of them.
It is unnerving to be out and have one stop firing. I lost two at one time on a flight to check my cattle about 12 miles from the airport. I was about 3 miles out and lost two, it was a slow steady decent at full throttle position. I made the end of the runway with about 50 feet to spare. I do not temp the Mac Attack Demons anymore.