I just use generic shoe trees from Bexley. They don't fit perfectly—none will—but these fill out the shoe decently and give it some structure. You can use paper to fill out the rest of the shoe if you wish. In my thinking, even an imperfect pair of shoe trees is better than none at all.
As for polishing, I play it by eye. Depends on how neat you want them looking and how often you wear them. I use some mink oil renovator on mine every few wears to keep the leather soft.