Over the years I’ve gotten better at being able to kneel, but when I started it was excruciating for me, too, and to help I would pull the cushions off the couch and kneel on those. I also used the sturdy furniture in the living room to help pull myself up.
Gradually it got more bearable, and I got stronger in my core and upper body,too, which helps a lot. Then I transitioned to the dojo and would use a crash pad or stacks of mats, until another dojo had to borrow them, and I moved to the floor (which is also covered in mats).
When we are outside I rely on a yoga mat. The thicker the better, and this one from @gaiam (called the essential) is the thickest one I can find. And you can fold it on itself for even more layers.
Other options I’ve seen, but haven’t tried are these silicone yoga jelly knee pads, and this exercise balance pad by @prosourcefit which can be used to make yoga poses more challenging, but also as a cushion for sitting, kneeling, etc.
Kneeling mats are everywhere, you can find them geared towards gardening, or even the auto garage!
Some of these may still be too hard, so you might have to get creative.
What these pads/mats/cushions all have in common is they help to dissipate the pain. Just like trying to use more of my shin when the pain gets to intense on the knee.
Do you use anything for cushioning when kneeling?