The PX28 is/was a silver-oxide battery. The alkaline versions are inferior for use in most photographic uses, because of the discharge curve.
Here is one on Amazon right now: http://www.amazon.com/Kodak-PX28-Bat.../dp/B00009R6T2
They are apparently still used in great numbers in dog collars. Here is another link: http://24hourbatteries.com/shop/px28battery__340.html
I buy them in quantity over the internet, because I have Mamiya cameras/accessories that use them. They were also used in Canon AE1's and A1's (among others).