you know the zookeeper started as a DIY unit right?
1 piece of 6" or 8" PVC pipe cut to whatever length you want to make the thing. They quote 7.25" diameter, but I don't know if that is the pipe or the fittings on the outside. Either way, you can buy 6" or 8" pipe, but it's in 10' sections and it isn't cheap.
Put a cleanout adapter on the bottom so you can unscrew it to get them out. Get a pipe cap, cut a hole in it, put an appropriately sized funnel in and slit the sides, drill a bunch of small holes to drain water, and you're good to go. Not quite as fancy, but it is what it is. Cost is probably about $20 cheaper than a new one, maybe $30 and that is assuming you have all of the tools and primer/glue for the PVC. I'm sure you can find the screw on rings for the pipes they are using, but I'm not sure where.
Also fwiw, a 5 gallon bucket with some holes drilled in it and the same type of slits cut in the lid works just fine as well. A lot less elegant, but still very functional