My stove takes 6mm stove knobs but I bought 8mm stove knobs.
Is there an adapter or something to make the 6mm shaft work with 8mm knobs?
Adapters like this actually do exist, but I have had a hard time tracking down the adapters by themselves. I have been able to find them packaged with replacement knobs, but am unsure if they will work with your particular one.
These sort of knobs are fairly inexpensive, and should fit your stove. They are available for electric and gas ranges.
I couldn't find such an adaptor, but you may be able to make your own. Find a section of small copper pipe with 1mm thick walls, with ID greater than 6MM and OD greater than 8MM. Cut a length to match the D-shaped section of shaft, and then squash it to a tight fit around the shaft. You'll probably need to cut it along the length to reduce some of the circumference; do that on the side opposite the flat of the D (that's where the stress is). Then see if you can get the knob to be a straight fit. It will be iffy, and may not last, as indeed there will be a lot of stress on that flat.