I didn't come up with anything on google, but this ome says they already have 1,000 Abrams tanks:
Security and Defense: Growing threats http://www.jpost.com/Features/FrontLines/Article.aspx?id=236377
Maybe a search of the Defense Appropriations committee (or whatever the proper name is) website would show something. Or, a search on the site where they keep full text copies of all the bills. I guess something like that could be a line item anywhere.
Another thought, give your Congressman's office a call....