I have an assortment of knee pads for public skating. My favourites are made by Xion and have d30 inserts. They do not prevent pain, but they do prevent bruising, and serious pain. They are expensive but seriously well made. That said they are not as protective as hockey pads. I have have some ForceField knee tubes but I do not recommend them. They are too hard, and when I fall, I get bruises from hitting the pad inserts! Some people wear softer pads offering some protection, basically removing the nastiness of a fall. The best are probably skate board pads with hard plastic covers and thick foam inserts, but they are big and not conspicuous. I've worn hockey pads but they are too bulky and look silly.
There are many pads with d30 and lookalikes such as POC and G-Form. The last one offers less protection, but they are still worth having and they are discreet and cheap. I have G-Form elbow pads which cost £35 including postage, and although they don't offer much protection, they offer enough to take away the worst of the pain, you know the kind you get when an elbow hits a solid surface! I guess you can Google to find reviews of knee pads, then find the best deal. The last seasons model is usually cheaper. Of course a problem is finding a good fit. I had to return some G-Form elbow pads as the sizing chart was wrong.