I don’t have an answer either, but I too began exploring ollas as a sustainable way to irrigate my gardens a couple of seasons ago. I tried milk jugs with pin holes this past year, but they stopped working mid-summer. I have not had a chance to dig them up and examine them., but I’ll finish my experiment next spring.
I found some large clay ollas both online and in local garden shops available for purchase, however they were very expensive. Buying in large quantities made them cost prohibitive, so I started my research and picked up some porous clay to attempt making them on my own. I have a kiln, but it’s very small, so I’ll likely pit fire them. One of my info resources is gentleman on YouTube named Andy Ward. He specializes in teaching ancient pottery fabrication techniques. When I reached out to ask him about ollas he was very helpful and guided me in selecting the right kind of clay for the project and put firing, etc. You might want to reach out to him there. (Update: I went looking for a link to his YouTube channel to share below and discovered he recently made a video on the subject. I haven’t seen it yet, but my guess is that it’s an excellent resource.)
Good luck!