What's the etiquette on asking pet owners for materials if you're puppy sitting?

The expectation is the dog or puppy will come with whatever is needed for the stay. In this example, the owners already told you they'd bring the food, pads, and crates.

For future dogs/puppies, usually during the meet&greet, you can discuss what you'd like them to bring. Some sitters create checklists and send it via Rover message. Food must always be provided because if the food is changed puppy or dog can have GI (stomach, etc) upset, which is unpleasant for all. Medicine if required must also be provided. I expect the dog will come with their harness, which fits them and their tags. Everything else can be discussed and may differ. I ask for a comfort item. Ideally their bedding to snuggle in to sleep/nap or even an unwashed item of theirs (sometimes it's a toy, a blanket, sometimes owner's clothes) . If they used a fixed length leash, I'll use that too. If they use retractable, I immediately change it with one of my spare fixed leashes for safer walks.

Not all pets are crate trained and not all pets are trained to go on pads. So, if you're sitting one that is not using these, there's no crate/pads to bring. Some owners will bring their dishes. It's good to be prepared to have items that you might need if they're not supplied.

During the M&G, it is very normal to discuss what the owner needs to bring with their dog. You should not feel uncomfortable requesting the basics and making suggestions for other "comfort" items the dog may like. At a minimum, the dog owner must supply food, medications (if needed), and a collar or harness with a leash. I will then make suggestions about other items they may want to include, such as a bed/crate if that is what the dog would prefer, treats, favorite toys, winter gear, bowls, etc. If they think the dog might exhibit some separation anxiety, I too will often suggest an unwashed T-shirt that the owner has worn. But I do tell them these items are optional. I do have extra bowls and beds, plus toys and treats. The day before the dropoff, i will message the owner with a reminder list of things to bring.

I do not accept puppies or dogs that use puppy pads. Since this puppy is being trained, I would expect the owner to provide whatever equipment he/she is using so that you can continue the process.

I send a To Bring list the week prior to the scheduled stay. The items are divided into three groups: Required, Recommended, and "Please Leave These At Home".