You can find this kind of info at the source . . . the Census Bureau's website:
Census Bureau Home Page. The drawback is that there is a lot of material here, so it can be difficult to find the page you want. It's often more useful to do a search at the website (for example, using a term like "county population projections" for the info you want) than to go by the table of contents, because the table of contents can lead you all over the place among the many pages at the website.