Have you looked at ski wear? I have some Mamut soft shell pants (very old) that have the ankle protectors (for crampon use). I got them at Sierra Trading Post. But that was years ago. I used to sew a seat patch on all my pants to make them wear longer and stay dry when I sat down on the ground. The patch is sort of heart-shaped.
Perhaps the first step is to get some cheap men's pants from the thrift shop and experiment with them. No sense in buying expensive pants and then ruin them by practicing on them. Make yourself a prototype using a long stitch so you can easily undo the seams and use the pieces for a pattern.