Hi, I entered in the US in July 2006 and left in July 2007 under F-1 visa (student status). I came baek to the U.S. in August 2008 under J-1 visa as trainee. To be eligible for non-resident alien status, I found I should have filed Form 8843 for 2007 (I filed it for 2006). Could you please advice whether I can still fine Form 8843 for 2007?

If you filed nonresident tax return for 2007, then you can file nonresident tax return for 2008.
IRS requires that a nonresident must file Form 8843. But a large number of nonresidents do not do this.

Just file the 2007 Form 8843 with your 2007 tax return.

If no 2007 return is required, then file the 2007 Form 8843 BY ITSELF.

Better late than never!