The other thing to consider is, even though your hotel will not let you in until 2, most will check luggage if you are trying to get another park day in. Arriving at 10 with all the parks opening at 9, you can almost have a full day.
As far as tickets, your best bet is the front gate or smarter online before you leave (CAA sometimes has deals). I have seen too many stories of people buying discounted tickets from ebay or a street vendor that don't work and the theme parks really don't care. The only other discounted tickets tend to involve a 6 hour time share presentation. Spend the extra $$ to make sure you have no issues going into the parks.