Walmart sometimes has dog pens, but you would have to make a top cover and they're not any cheaper there.
I have one but you're too far for me to give it to you
You could probably find one used. A lot of people buy dog pens (without the top) and you could just use some fabric or a heavy tarp as the top.
You could build something similar out of wood and chicken wire if you feel industrious. Actually they sell plastic chicken "wire" at walmart for garden fences, so that may work and it's pretty cheap. I don't think cats could get through it. It's made of the same stuff plastic screen is and none of my cats can rip my plastic screens.