I used to have the same issue. I find folding the back of the pants flat under the bottom strap provides more support and stops it slipping. As far as pads go, it's a question of budget, level of cricket and personal preference.
I have only had 4 pairs in 25 years and the best ones had Velcro inserts that allowed me to adjust the knee roll cup and insert for legs. Recently went for slighter set of GN x181 and they don't offer the same protection but are easier to run in.
Visit one of the sponsors shops and try some on.