This stand alone step stool is mainly for the sink and other counters. It's the tallest on the market, which means they can use it in places other than the bathroom. This gets dragged all over our house, in a great way. $27.
Little Looster Stool
This is slightly bigger than some of the potty step stools out there, which is a bummer to work around BUT, I found this to be invaluable if you have a boy. Because they're going to want something seriously solid to stand on while peeing. The other smaller ones I've seen just didn't cut it.
Trust me when I say, if you have a boy, you need a splash guard. This is the one we used because I wanted handles and a splash guard, but arm and hammer has apparently discountinued it. You can get it from these third party sellers for around $15 here.
Oh Crap, Potty Training
Absolutely could not have made it through potty training without this book. It made me laugh out loud repeatedly, and it made me believe I could do this. My son always could, it was me that needed the convincing. Normally I would suggest the library for this, but at least at BPL it's way on back order. Just get it on kindle for $13 here.
Elmo's Potty Book
You may want to get a book to explain potty training to your child. Out of all the books out there, this one is what my toddler reacted to the most. He loved the explanations and wanted to be "just like elmo". Plus, there are activities on each page, so it gave him something to do while he was on the potty. They have lots of these used for very cheap, so you can get one for the price of shipping.
Pirate Potty Book
This was a secondary favorite potty book about using the potty. Not as popular as Elmo, but it gave me a break from reading Elmo for the millionth time. The only catch is that your kid will proooobably make you read it in a pirate voice. $5 for the paperback.