I have found that for myself, and many other ladies whom are a part of my league that 187's Pro Knee Pads. These are pads that skateboarders use so the knee caps are pretty indestructible. But, if you do break one 187 will replace them. Sin City Skates (http://www.sincityskates.com), a popular derby website says:
"The cap "pour point" is also on the side (instead of on the top, where there is already a lot of stress) for these, so they are much harder to crack, and easier to replace if they do."
Compared to some other knee pads, these may seem expensive at $65.00 but I would not skate with any other knee pad. The pro-knees have a thicker padding that is great for impact but the fabric is also a "quick dry" material, making washing easier.
When I first made the switch I didn't think that I would like them since they are thicker than other pads; I thought cross-overs might be harder but after a lap or two I was ready to go! I did not feel the falls as much as I had before and actually felt like I could control my knee falls, being able to pop up more easily.
Check out all of 187s gear at http://www.187killerpads.com/