If you look at a map find Commerce and Cherry Street just North of the Alamo Dome. Then head North on Cherry. There will be plenty of parking in this area. This is where most of the tailgating is going on. It is a short walk to Sunset Station and to the Alamo dome.
Parking will cost $40 at Commerce and Cherry but should be in the $10 to $20 range as you head North.
Another alternative is to find a parking garage downtown and either walk or take a taxi.