SOP after buying a pet at an expo?

I am going to a reptile show tomorrow with the intention of buying a snake if they have what I am looking for. I want to check out the whole show regardless. If I find what I am looking for early on, is the expectation that I carry it around for an hour? Is it precedented to purchase an animal and come back for it later? Is there some system of calling dibs implemented by sellers? Am I meant to wait till the end of my attendance and just hope availability is unchanged? Is it considered best practice just to call it a day after making a purchase?

As an aside, vendors will typically accept $100 bills, right?

Thanks in advance. If you have any other relevant tips you think would be helpful, please let me know.

Hundreds are typically fine.

Buying early and asking the vendor to hold until you're ready to leave is common, and is a good practice. Vendors will happily put your name on the animal and hold it under the table.