I could have bought another set, but instead decided to use sealed lead acid batteries. I have a number of them, and usually have a couple charged at any given time. Since they are 7AH batteries and my radio only draws about 200ma full volume, they go a long time between charges. The cassette tape deck would draw more power, but I don’t use that.
Sealed batteries are often used in emergency lights. I was able to get a bunch for free when they were replaced in a building I work in. Code requires regular replacement, and they have a lot of life left when replacement time comes. You can also buy them new. The one I have is fairly heavy, but in my use I don’t have to move it around much.
The 6 D cells provide 9V volts when fully charged. The sealed battery is a 12V model and actually has a voltage of around 13.6V when fully charged. To avoid damage to the radio a regulator is required. I used a commercially available regulator kit. You can build your own with a LM7809 or LM7808 and a few capacitors. Just use the example circuit in the data sheet.