I've used
the Davis single knee guards, but found them to be too wide for my preference. They require full-fledged plate pants (i.e. even combos are tight). I've only used them a couple of times. They're for sale if anyone is interested.
My current pair are
the ITECH guards, which fit nicely under even base pants, yet have good air circulation and excellent protection. The hard plate floats above your leg on an inner lining. There is an air gap between the guard and the lining, and plenty of holes for air to circulate.
My favorites were the earlier version of the ITECH guards, which had the Cooper name, and did not have the protective plate above the joint between the shin and kneecap. I liked them better because they were even a better fit under ordinary (base) pants. I could cut the extra plate off, but I haven't yet. I bequeathed my Coopers to my daughter. I can't find the original Coopers anymore.
You can find links to various equipment dealers in the Links section of
Softball Umpires.