They trained late and are tall, so I know what you mean about the urine leaking out. It doesnt happen in this seat, and its incredibly inexpensive, and my kids LOVE it!! I bought about 3 different seats/chairs before they fell in love with this one. Its easy to clean too.
I bought mine at Babies R Us, but theyre available at Amazon and all the other big boxes, I believe.
Good luck! I have found that my kids only trained when they were ready. So I let them tell me when they were ready, and Ive never had a struggle with it. At times, when they dont feel like stopping to go before we leave the house, I offer a reward like organic fruit snacks (I call them fruit candies!!)
We also have a toilet seat that has both the big and small seat, and DS likes that. THe small seat makes him feel secure. He still wont go #2 in a public toilet because theyre too large and they frighten him.
Heres the toilet seat. You can get it in oval or round.