I tried starting a thread on t-shirts several years ago and it died immediately. Let's see if your interest will revive it.
Zoo T-Shirts - ZooChat
Oh I will add to this most definitely. My wife and I have visited roughly 16 zoos in the last year and our thing is we buy a shirt from them. Most of the zoos we have been to have a fairly decent selection of shirts, but if you look online, you are not going to see anything remotely close to what they have in person. I believe this is due to what others have said in this thread that there is one company that provides a merchandising service for a number of zoos here in the States.
This has also made me a little frustrated in the miniature animal selection at these gift shops. Back in the 90s, you could find all sorts of high quality animal figures, but now mostly are the cheap ones. In my opinion, the best gift shops still have the Schleich or Safari figures. But alas, it seems many shops have sold out to the plush crowd.
I also too wish that more shops here in the US have their own guides or history books in stock. I prefer to buy directly from them, especially in a covid year, but I have had to go online to find most of them. I usually wait until after I visit the zoo to make sure they do or don't have the books before I order it.