I just went to Target and picked up the cheapest, just make sure the cup insert is easily removed. If it "slides"into a slot, make sure it doesn't tend to stick. Ours, you lifted the lid and the cup lifted out easily. Then you poured it into the toilet and said "bye-bye pee-pee (or poo-poo)" and taught them to flush. Have to say, when they get taller and want to use the big toilet, we found a seat that had two seats and the lid. The middle or second seat had the smaller opening and helped my kids feel relaxed. My oldest is tall and 5 now, but still feels more relaxed using this seat.
Now for on the go, do pick up a fold out seat converter so that she doesn't develop the fear of falling in. I think I found mine at Target again for about $10. It was a life saver for those public toilet needs.
Happy training!
K.