There's a C-shaped piece in the hole through the bolt near the right (inner) end. Press one leg of that toward the edge of the door (the latch). Normally there's a formed sheet metal shaft with an opposing C shape going through there that rotates and does just that.
I don't ever recall seeing a "kit" for this. When I've had to do it, I just got the two knobs and cut the head off one of the screws. If the screws that came with the knobs were too short, then I got a longer screw and cut the head off. There are double threaded studs out there but they are usually too big for what you want to do.
I have a similar lock. Press straight down in the center of the hole with an awl while wiggling the knob and pulling gently. There is a small spring loaded pin that has to be depressed to remove the knob
I think you might be referencing what is commonly called an edge pull. There are many manufacturers, each with their own selection of styles and finishes. This one (not an endorsement) is made by a fairly steady cabinet hardware company called Amerock:
remove the control handle then unscrew the octagonal cover this should reveal the valve top.
take photos and measurements. go to the hardware store and buy as washer kit to rebuild the valve
Now turn ff the water supply (or supplies)
turn the valves on and make sure that the is not a fast stream of water.
now remove the top part of the valves (it unscrews)
If you want a nice clean solution then you want to check out a selection of hand wheels. Some types even have a crank built into them. Many even have a provision for a set screw to lock it to the shaft to which they mount. There are styles made of various types of materials. Those shown here are made of aluminum.
There are many variations on this answer, allowing you to fine tune the results to your liking.
If you have a disk cut from thin plywood or similarly flat material and tighten your locking nuts on each side, you have a knob of sorts. The larger the diameter of the disk, the greater control you will have, as the resulting lever become longer.
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