There are a couple of car parks on Winwick Street just near Central Station. carry on past the stadium, turn right at the next lights (original Wire pub on your left), take first left at the roundabout at the Rodney, through the next lights and there are 2 pay and display car parks on your right.
200 yards max from ground and about £2-60 for parking.
Loads of on free on-street around if you know where to look but I'm not telling you where.