hi peoples have a prominent tibial tuberosity from old osgood schlatters (look that up on dr google haha) gets pretty tender with all my kneeling as a newbie
What board do you have? Can you put more foam on it?
also what are air and water temps. are you wearing rubber? Last summer I wore a farmer John suit just to have some rubber on my knees. A buddy was wearing his biking knee pads
Shove closed cell sleeping mat foam down inside your wetsuit over your knees, it's pretty easy to do as you are putting on the wetsuit and doesn't move once the wetsuit is on - was a game changer when I was learning
My advice... ask around at your local spot to see if anyone has some you can borrow. It won't be that long before you don't need them anymore. If you're set on buying some, the best ones I found were from the sailing industry.
I have super sensitive knees, likely from stupid stuff as a kid, I just wear the cheap knee pads from the local hardware store. They last about a season, then I replace them. Wife wears leggings, so she stuffs closed cell foam in to her leggings.