Glen-- Watch for Edmunds press releases. We put out a sales forecast press release about a week before actual sales numbers are released, which will give you some help on what to expect when actual sales numbers are released. The most recent example is the June sales forecast.
There aren't many companies that will give you monthly auto sales info for free. Wards is a great subscription service. AutoData is also subscription, but they will give you current month sales for free on their home page. You can also check the "US Sales Reporting Calendar" on AutoData's home page to see when the automakers will publicly report their monthly sales-- announced via press release. Like Sylvia said, press releases from the automakers are your most immediate resource for getting auto sales data. Wards and AutoData will always have lag time of hours or even a day before they post sales for all the automakers.
vivianzhao-- gasgoo is focused on the Chinese auto industry, and the data and stats at the link provided are for the Chinese auto industry. While they do re-post world news on their news page, the available info is limited. To follow news and analysis of the US auto industry, US media outlets such as AutoObserver.com are better resources.